fix(worktree): clean error when restoring/promoting onto an occupied branch

[posthog]

Problem

Restoring a suspended task (or promoting a local task to worktree mode) failed with a raw git worktree add exited with code 128 error whenever the task's branch was still checked out in another worktree. createWorktreeForExistingBranch only verified the branch existed and the worktree name/path was free — never whether the branch was already live elsewhere — so git bailed and the raw failure escaped through the restore/promote paths into error tracking as an uncaught exception.

The workspace creation path already caught the "is already used by worktree" message and turned it into a friendly error, but the restore (suspension/unarchive) and promote paths did not. This hit the team dogfooding PostHog Code — low volume but real, and the raw exception spammed error tracking even though the restore UI already degrades to a helpful toast.

Changes

Detect the occupied branch up front in createWorktreeForExistingBranch and throw a clean, actionable message (Branch '<x>' is already used by worktree at '<path>') instead of letting the opaque git exit-128 error surface. The wording deliberately keeps the "is already used by worktree" substring, so the existing consumers that key on it — the workspace creation path and the restore toast — keep recognising the case unchanged.

Fixing it at this shared chokepoint gives the restore and promote paths the same occupied-branch handling the creation path already had, and also covers the archive/unarchive path, in one place. The check degrades gracefully: if the worktree list can't be read, it falls through to the prior behaviour rather than blocking creation.

How did you test this?

• Added a unit test asserting createWorktreeForExistingBranch throws a clean occupied-branch error (matching is already used by worktree, and not exited with code 128) when the branch is already checked out.
• pnpm --filter @posthog/git typecheck and biome lint on the changed files pass.
• The git-fixture tests in this file can't execute in the automated environment (it blocks git commit, which every fixture's beforeEach seed relies on — pre-existing tests fail the same way); the new test mirrors the existing lifecycle tests and runs under CI's real git.

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit ed1e178.}

[github-actions]

This PR has had no activity for 7 days and has been marked stale. We are moving to the monorepo and tightening PR staleness in preparation, so it will be closed in 7 days if no further activity occurs.