feat: record base SHA at gw new --stack for a deletion-proof restack

src/commands/new.rs

@@ -142,6 +142,12 @@ pub fn run(branch_name: Option<String>, stack: bool, verbose: bool) -> Result<()
     // guard work to handle; `gh pr create` errors loudly on a missing base.)
     if let Some(base) = &pr_base {
         git::set_branch_base(&branch_name, base, verbose)?;
+        // Record the base tip SHA (= the fork point, since --stack branches off
+        // the current HEAD) so a later restack can `rebase --onto` even if the
+        // base branch has since been deleted.
+        if let Ok(sha) = git::head_commit() {
+            git::set_branch_base_sha(&branch_name, &sha, verbose)?;
+        }
     }
 
     if behind_count > 0 {

src/commands/status.rs

@@ -117,6 +117,11 @@ pub fn run() -> Result<()> {
             recorded_base_merged = check_base_pr_merged(base).is_some();
         }
     }
+    // The recorded base tip is the `rebase --onto` boundary that survives the
+    // base branch being deleted (used when the base merged before this PR).
+    let recorded_base_sha = recorded_base
+        .as_ref()
+        .and_then(|_| git::branch_base_sha(&current));
 
     // Stash count
     let stash_count = git::stash_count();
@@ -135,6 +140,7 @@ pub fn run() -> Result<()> {
         base_pr_merged: base_pr_merged.as_deref(),
         recorded_base: recorded_base.as_deref(),
         recorded_base_merged,
+        recorded_base_sha: recorded_base_sha.as_deref(),
     });
     next_action.display(&current);
 

src/git/mutation.rs

@@ -173,17 +173,33 @@ pub fn set_branch_base(branch: &str, base: &str, verbose: bool) -> Result<()> {
     )
 }
 
-/// Clear a branch's recorded base (`branch.<name>.gwBase`).
+/// Record the base tip SHA a branch was stacked on (`branch.<name>.gwBaseSha`).
 ///
-/// A no-op (not an error) when the key is absent, so callers can clear
-/// unconditionally — e.g. `gw sync` after restacking a branch onto the default
-/// branch, where it is no longer stacked.
+/// A `git rebase --onto` boundary that survives the base branch being deleted.
+pub fn set_branch_base_sha(branch: &str, sha: &str, verbose: bool) -> Result<()> {
+    git_run(
+        &["config", &format!("branch.{branch}.gwBaseSha"), sha],
+        verbose,
+    )
+}
+
+/// Clear a branch's recorded base info (`branch.<name>.gwBase` and `.gwBaseSha`).
+///
+/// Each unset is a no-op (not an error) when the key is absent, so callers can
+/// clear unconditionally — e.g. `gw sync` after restacking a branch onto the
+/// default branch, where it is no longer stacked.
 pub fn unset_branch_base(branch: &str, verbose: bool) -> Result<()> {
+    unset_config_key(&format!("branch.{branch}.gwBase"), verbose)?;
+    unset_config_key(&format!("branch.{branch}.gwBaseSha"), verbose)
+}
+
+/// `git config --unset <key>`, treating "key absent" (exit 5) as success.
+fn unset_config_key(key: &str, verbose: bool) -> Result<()> {
     if verbose {
-        output::action(&format!("git config --unset branch.{branch}.gwBase"));
+        output::action(&format!("git config --unset {key}"));
     }
     let output = Command::new("git")
-        .args(["config", "--unset", &format!("branch.{branch}.gwBase")])
+        .args(["config", "--unset", key])
         .output()
         .map_err(|e| GwError::GitCommandFailed(format!("Failed to execute git: {e}")))?;
     // Exit code 5 = "key was not present"; treat as already-clear.

src/git/query.rs

@@ -107,6 +107,16 @@ pub fn branch_base(branch: &str) -> Option<String> {
     if value.is_empty() { None } else { Some(value) }
 }
 
+/// Read the recorded base tip SHA for a branch (`branch.<name>.gwBaseSha`).
+///
+/// Recorded by `gw new --stack` as the parent's HEAD at stack time. Serves as a
+/// `git rebase --onto` boundary that survives even if the base branch ref is
+/// later deleted (e.g. cleaned up after its PR merged). `None` when unset.
+pub fn branch_base_sha(branch: &str) -> Option<String> {
+    let value = git_output(&["config", "--get", &format!("branch.{branch}.gwBaseSha")]).ok()?;
+    if value.is_empty() { None } else { Some(value) }
+}
+
 /// Get the current HEAD commit hash
 pub fn head_commit() -> Result<String> {
     git_output(&["rev-parse", "HEAD"])

src/state/next_action.rs

@@ -34,8 +34,13 @@ pub enum NextAction {
     /// Base PR was merged, should sync (update base to main, rebase, push)
     SyncNeeded { base_branch: String },
     /// Recorded stacked base merged before this branch's PR was created; rebase
-    /// onto the default branch and open a normal (non-stacked) PR.
-    StackedBaseMerged { base_branch: String },
+    /// onto the default branch and open a normal (non-stacked) PR. `base_sha` is
+    /// the recorded base tip used as the `--onto` boundary when present (it
+    /// survives the base branch being deleted).
+    StackedBaseMerged {
+        base_branch: String,
+        base_sha: Option<String>,
+    },
 }
 
 /// Inputs for [`NextAction::detect`]. Bundled into a struct because the detected
@@ -56,6 +61,9 @@ pub struct DetectContext<'a> {
     /// The recorded base's PR has already merged (stale stacked base): the
     /// branch should rebase onto the default branch and open a normal PR.
     pub recorded_base_merged: bool,
+    /// Recorded base tip SHA (`gw new --stack`); the `--onto` boundary that
+    /// survives the base branch being deleted.
+    pub recorded_base_sha: Option<&'a str>,
 }
 
 impl NextAction {
@@ -70,6 +78,7 @@ impl NextAction {
         let base_pr_merged = ctx.base_pr_merged;
         let recorded_base = ctx.recorded_base;
         let recorded_base_merged = ctx.recorded_base_merged;
+        let recorded_base_sha = ctx.recorded_base_sha;
         // On home branch
         if current_branch == home_branch {
             // Behind upstream → sync first
@@ -131,6 +140,7 @@ impl NextAction {
                 if let Some(base) = recorded_base {
                     return NextAction::StackedBaseMerged {
                         base_branch: base.to_string(),
+                        base_sha: recorded_base_sha.map(String::from),
                     };
                 }
             }
@@ -244,7 +254,10 @@ impl NextAction {
                 println!();
                 println!("  gw sync");
             }
-            NextAction::StackedBaseMerged { base_branch } => {
+            NextAction::StackedBaseMerged {
+                base_branch,
+                base_sha,
+            } => {
                 output::action(&format!(
                     "Next: base '{}' merged — rebase onto main",
                     base_branch
@@ -253,10 +266,13 @@ impl NextAction {
                 // `--onto` replays only THIS branch's commits. A plain
                 // `git rebase origin/main` would re-apply the (squash-)merged
                 // base's commits too, producing a doubled/conflicting diff.
+                // Prefer the recorded base SHA: it still resolves even though
+                // the merged base branch has likely been deleted.
+                let onto = base_sha.as_deref().unwrap_or(base_branch);
                 println!("  git fetch --prune");
                 println!(
                     "  git rebase --onto origin/main {}  # replay only your commits",
-                    base_branch
+                    onto
                 );
                 println!("  # then open a normal PR (base is now main):");
                 println!("  gh pr create -a \"@me\" -t \"...\"");
@@ -310,6 +326,7 @@ mod tests {
             base_pr_merged: None,
             recorded_base: None,
             recorded_base_merged: false,
+            recorded_base_sha: None,
         }
     }
 
@@ -442,7 +459,32 @@ mod tests {
         assert_eq!(
             action,
             NextAction::StackedBaseMerged {
-                base_branch: "feature/parent".to_string()
+                base_branch: "feature/parent".to_string(),
+                base_sha: None,
+            }
+        );
+    }
+
+    #[test]
+    fn test_recorded_base_merged_carries_recorded_sha() {
+        let action = NextAction::detect(&DetectContext {
+            recorded_base: Some("feature/parent"),
+            recorded_base_merged: true,
+            recorded_base_sha: Some("abc1234"),
+            ..ctx(
+                "feature/child",
+                "main",
+                &WorkingDirState::Clean,
+                &SyncState::Synced,
+                None,
+                true,
+            )
+        });
+        assert_eq!(
+            action,
+            NextAction::StackedBaseMerged {
+                base_branch: "feature/parent".to_string(),
+                base_sha: Some("abc1234".to_string()),
             }
         );
     }

tests/new_test.rs

@@ -108,6 +108,12 @@ fn test_stack_branches_off_current_branch() {
         run_git(dir, &["config", "--get", "branch.feature/child.gwBase"]),
         "feature/parent"
     );
+    // The base tip SHA is recorded too (the rebase --onto boundary that
+    // survives the base branch being deleted) and equals the parent's HEAD.
+    assert_eq!(
+        run_git(dir, &["config", "--get", "branch.feature/child.gwBaseSha"]),
+        parent_head
+    );
 }
 
 #[test]