fix: stop replayed bootstrap turns from re-running git worktree add

[patroza]

Problem

Every worktree-backed thread creation showed a "Queued message was rejected: Git command failed in GitVcsDriver.createWorktree ... git worktree add failed" toast, even though the worktree was created successfully.

Server logs (redacted stderr is kept server-side) show the real error:

args: 'worktree add -b t3code/68a2fc69 /home/patroza/.t3/worktrees/t3code/t3code-68a2fc69 ba86864dc...'
stderr: "fatal: a branch named 't3code/68a2fc69' already exists"

Reflog timeline for one occurrence: branch created 16:00:24 (first bootstrap), duplicate worktree add failed 16:00:26, rename 16:00:28 — the same bootstrap turn was delivered twice, ~2s apart.

Root cause

ChatView persists every outgoing turn to the IndexedDB outbox before calling startThreadTurn, and removes it only after the call settles. A bootstrap turn stays in flight for several seconds (worktree creation, fetch, setup), and in that window the thread projection arrives with no running session — exactly the condition under which the outbox drain effect fires. The drain found the still-pending entry and re-sent the full turn, bootstrap included. On the server, thread.create deduped ("already exists" → resume remaining bootstrap), but prepareWorktree re-ran git worktree add -b against the branch the first delivery had just created, and the replay was rejected with the toast.

Fix

• Client (ChatView.tsx): track in-flight startThreadTurn message ids in a module-level set; the outbox drain skips turns whose original send has not settled.
• Server (ws.ts): when a replayed bootstrap reaches prepareWorktree and the thread already has a recorded worktree, reuse that worktree (refresh its git status) instead of re-creating it, and skip the duplicate setup-script launch. Fresh bootstraps are unaffected.

Verification

• New focused test: reuses the existing worktree when a replayed bootstrap already prepared it (replay with prepareWorktree → createWorktree not called, setup not re-run, turn dispatched).
• vp test run src/server.test.ts -t bootstrap → 11 passed; -t worktree → 3 passed.
• tsgo --noEmit clean for apps/server and apps/web.
• Integrated web pass (test-t3-app, isolated env): created a worktree-backed thread from the composer; thread + worktree + branch rename all succeeded with zero git command failed log entries and no rejection toast.

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