Staged attachments live in git-excluded .xum/user-attachments and are lost by snapshot archive

[ThomasK33]

Context

Workspace attachment staging writes uploads into the git-excluded .xum/user-attachments directory inside the checkout. PR #3940 pairs staging with archive admission (synchronous guard + preflight counter), which closes the live race between an in-flight upload and a model-driven snapshot archive.

Problem (Codex review round 19, P2 — placement leg)

Snapshot archives capture tracked/untracked git-visible state and then remove the managed worktree. Because .xum/user-attachments is git-excluded:

• A completed upload is absent from the snapshot, so after unarchive the persisted chat/draft attachment path points at a file that no longer exists.
• This is not a race — it is data placement: any staged attachment in a snapshot-archived workspace is silently lost.

Suggested directions

1. Store staged attachments outside the checkout (e.g. under the session dir, like other durable per-workspace artifacts), leaving a stable path that survives worktree removal/recreation; or
2. Include .xum/user-attachments in the snapshot capture and restore it on unarchive; or
3. On archive, migrate staged attachments referenced by persisted chats/drafts into the session dir and rewrite references.

Option 1 is likely cleanest for new writes but needs a migration/compat story for existing persisted paths (upgrade↔downgrade rule).

References

• Staging: WorkspaceService.stageAttachment → stageWorkspaceAttachment (.xum/user-attachments)
• Snapshot capture: worktreeArchiveSnapshotService
• Review thread: PR 🤖 feat: restore task_workspace_lifecycle with archive/unarchive for peer workspaces #3940, round 19 (src/node/services/workspaceService.ts)

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Generated with mux • Model: anthropic:claude-fable-5 • Thinking: xhigh

[linear-code]

MUX-119