docs: record how to run the documented inner loop from a tend session

.claude/skills/running-tend/SKILL.md

@@ -2,9 +2,10 @@
 name: running-tend
 description:
   PRQL-specific guidance for tend CI workflows. Adds a standing exception for
-  filing issues in other repos, PR title conventions, CI structure,
-  Dependabot-batch polling, weekly maintenance tasks, and issue-closing policy
-  on top of the generic tend-* skills. Use when operating in CI.
+  filing issues in other repos, PR title conventions, CI structure, which test
+  commands actually run inside the sandbox, Dependabot-batch polling, weekly
+  maintenance tasks, and issue-closing policy on top of the generic tend-*
+  skills. Use when operating in CI.
 ---
 
 # Running Tend in PRQL
@@ -42,6 +43,84 @@ permission first) still applies when the target shows no agent signals.
   in #5753, so bot PRs must be merged manually by a maintainer (or via repo
   branch-protection auto-merge if a maintainer enables it on the PR).
 
+## Running tests from a tend session
+
+**`cargo-insta` and `cargo-nextest` are not on the sandbox PATH by default**, so
+`task prqlc:test` and `task prqlc:pull-request` — both of which route through
+`cargo insta test --accept … --test-runner=nextest` — answer `no such command`
+out of the box. `.github/actions/tend-setup` installs both, but
+`baptiste0928/cargo-install` puts them under the runner user's home
+(`/home/runner/.cargo-install/<crate>/bin`), and tend derives the sandbox PATH
+without any runner-home directory.
+
+The binaries are world-executable and `/home/runner` is `drwxr-x--x`, so
+`tend-sandbox` can traverse to them without being able to enumerate that home.
+Prepend them in the same invocation as the command — shell state doesn't persist
+between tool calls:
+
+```sh
+export PATH="/home/runner/.cargo-install/cargo-insta/bin:/home/runner/.cargo-install/cargo-nextest/bin:$PATH"
+task prqlc:test
+```
+
+`task` is already on the sandbox PATH, so `task prqlc:test` — the inner loop
+`CLAUDE.md` documents — then runs unchanged. Verified in a session on
+2026-08-26: it exits 0 in ~3 minutes on a warm `target/`, and
+`cargo insta test --accept` rewrote an `assert_snapshot!(ident, @"")` literal in
+place, so the initialize-empty-then-accept flow works here too.
+`task prqlc:pull-request` does not: through `test-all` it also needs
+`cargo-llvm-cov` for the coverage step and `pre-commit` for `:lint`, and
+`tend-setup` installs neither. Those two steps are all that's missing, so when
+`CLAUDE.md` calls for the pre-return check, run the rest by hand: `test-all`'s
+first two commands — the `--features=default,test-dbs` insta/nextest run and the
+matching `cargo test --doc`, both over `packages_core` plus `packages_addon` and
+`packages_bindings` — and then `:lint`'s
+`cargo clippy --all-targets --all-features`, budgeting for the cold-cache cost
+the `rust-toolchain.toml` section below describes. Don't substitute
+`prqlc:test`: it covers `-p prqlc-parser -p prqlc` only, so a change that breaks
+`prqlc-js` or `prqlc-c` passes it.
+
+Notes on running tests here:
+
+- **Never verify with `INSTA_UPDATE=always cargo test`.** `always` selects
+  insta's in-place update, so a `.snap` file is rewritten to whatever the code
+  produced and the assertion passes unconditionally — a green run that checked
+  nothing. Use it only to regenerate file snapshots deliberately, then re-run
+  plain `cargo test` to verify. Plain `cargo test` is a real check: insta's
+  default `auto` behaviour writes nothing when `CI` is set.
+- **A green `task prqlc:test` is weaker than it looks.** The task runs
+  `cargo insta test --accept` and then
+  `cargo clippy --fix --allow-dirty --allow-staged`, so exit 0 can mean
+  snapshots and source were rewritten to match what the code now produces rather
+  than that the assertions held. Check `git status` and read the diff before
+  reporting the run as verification.
+- Without the export, plain `cargo test` is the fallback — scoped the same way
+  the `CLAUDE.md` examples are, e.g.
+  `cargo test -p prqlc --test integration -- date`. Inline snapshots then have
+  to be transcribed by hand: insta never rewrites a source file itself, it
+  records the value in a pending-snapshot file and leaves applying it to
+  `cargo-insta`. Take the expected value from the failure's diff, write it into
+  the `@"…"` literal, and re-run to confirm.
+- **Don't `cargo install` either crate.** Building from source costs several
+  minutes of the session budget, and the binaries `tend-setup` already built are
+  one `export` away.
+- Scope the claim to the command that actually ran. `cargo test -p prqlc` is not
+  `task prqlc:pull-request`, and saying so is the difference between a useful
+  caveat and a false green.
+
+**On making the export unnecessary — a maintainer's call.** #6144 (a workflow
+step symlinking both binaries into `/usr/local/bin`) sat open for 20 days and
+was closed unmerged on 2026-08-25; don't re-propose it. `sandbox_path:` is not
+the alternative either: tend's `proxy/setup-sandbox.sh` rejects any entry under
+the runner's home outside the checkout and fails the job with "Install the tool
+into the sandbox with `sandbox_setup:` instead" — deliberately, since that home
+can hold credentials unrelated to the tool being reached for. `sandbox_setup:`
+is the supported lever, and it needn't mean a rebuild: `tend-setup` runs before
+the tend action, so copying the two existing binaries into
+`/home/tend-sandbox/.cargo/bin` (already on the sandbox PATH, and writable by
+the sandbox user) would cost a `cp`. Leave that to a maintainer rather than
+re-litigating the area from a session.
+
 ## Verifying a `rust-toolchain.toml` bump
 
 The `update-rust-toolchain` action opens `build: Update rust toolchain version`