feat(workspace): auto-unlink linked branches that no longer exist

[adboio]

Problem

Nothing ever cleans up a task's linkedBranch when its branch is deleted — the usual end state after a PR merge. Returning to a finished task and asking a follow-up from main triggers the wrong-branch warning on every send, about a branch that can never be checked out again. Production data suggests this stale-link case drives much of the ~59% "continue anyway" rate on the mismatch dialog (1,249 dialogs in the last 60 days).

Change

getWorkspace now self-heals stale links:

• New anyBranchRefExists query in @posthog/git: true when the branch exists as a local head or a remote-tracking ref on any remote (for-each-ref refs/heads/<b> refs/remotes/*/<b>). No network — a deleted remote branch stops counting once the user's refs know about it; tags and raw commit-ishes don't count.
• When the linked branch mismatches the current branch and no ref remains, the link is dropped via the existing unlinkBranch, with a new "auto" source on the Branch unlinked analytics event.
• Being on the linked branch proves it exists, so only mismatched reads pay the extra ref lookup; an in-flight guard keeps concurrent reads from stacking git calls, and the linked state is re-read after the git check to avoid racing a concurrent re-link.

Also makes the mock workspace repository's updateLinkedBranch persist (was a no-op) so service tests can exercise link state.

Tests

• workspace.test.ts: parameterised refExists → linkedBranch outcome, focused side-effects test (event + analytics + persisted null), skips the check when on the linked branch, keeps the link if the git check throws.
• queries.test.ts: anyBranchRefExists local ref / remote-tracking ref on a non-origin remote / missing branch / tag-only name (plumbing-based fixture, no git commit needed).

Stack:

1. → this PR — auto-unlink stale linked branches
2. feat(workspace): replace branch mismatch modal with non-blocking banner #3183 — replace the modal with a non-blocking pre-send banner (also deletes the dialog whose Switch action was broken; that fix was fix(workspace): make branch mismatch "Switch branch" actually send the message #3179, closed unmerged in favor of this stack)

🤖 Generated with Claude Code

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit ed27d0a.}

[greptile-apps]

_{Reviews (1): Last reviewed commit: "feat(workspace): auto-unlink linked bran..." | Re-trigger Greptile}

[greptile-apps]

The "unlinks when no refs left" and "keeps when a ref exists" cases follow the same structural pattern — seed a task, mock anyBranchRefExists, call getWorkspace, assert linkedBranch — and the team rule is to always prefer parameterised tests. The extra assertions on the false case (event, analytics, DB persistence) can live in their own focused test so the core "refExists → linkedBranch outcome" logic is expressed once.

Suggested change

	it("unlinks a mismatched branch that has no refs left", async () => {
	seedLinkedTask("feat/gone");
	vi.mocked(anyBranchRefExists).mockResolvedValue(false);
	const emitted = vi.fn();
	service.on(WorkspaceServiceEvent.LinkedBranchChanged, emitted);
	const workspace = await service.getWorkspace("t1");
	expect(workspace?.linkedBranch).toBeNull();
	expect(emitted).toHaveBeenCalledWith({ taskId: "t1", branchName: null });
	expect(mocks.analytics.track).toHaveBeenCalledWith(
	ANALYTICS_EVENTS.BRANCH_UNLINKED,
	expect.objectContaining({ task_id: "t1", source: "auto" }),
	);
	expect(mocks.workspaceRepo.findByTaskId("t1")?.linkedBranch).toBeNull();
	});
	it("keeps a mismatched branch that still has a ref", async () => {
	seedLinkedTask("feat/alive");
	vi.mocked(anyBranchRefExists).mockResolvedValue(true);
	const workspace = await service.getWorkspace("t1");
	expect(workspace?.linkedBranch).toBe("feat/alive");
	expect(mocks.analytics.track).not.toHaveBeenCalled();
	});
	it.each([
	{ branch: "feat/gone", refExists: false, expected: null },
	{ branch: "feat/alive", refExists: true, expected: "feat/alive" },
	])(
	"linkedBranch is $expected when refExists=$refExists",
	async ({ branch, refExists, expected }) => {
	seedLinkedTask(branch);
	vi.mocked(anyBranchRefExists).mockResolvedValue(refExists);
	const workspace = await service.getWorkspace("t1");
	expect(workspace?.linkedBranch).toBe(expected);
	},
	);
	it("emits event, tracks analytics, and persists null when auto-unlinking", async () => {
	seedLinkedTask("feat/gone");
	vi.mocked(anyBranchRefExists).mockResolvedValue(false);
	const emitted = vi.fn();
	service.on(WorkspaceServiceEvent.LinkedBranchChanged, emitted);
	await service.getWorkspace("t1");
	expect(emitted).toHaveBeenCalledWith({ taskId: "t1", branchName: null });
	expect(mocks.analytics.track).toHaveBeenCalledWith(
	ANALYTICS_EVENTS.BRANCH_UNLINKED,
	expect.objectContaining({ task_id: "t1", source: "auto" }),
	);
	expect(mocks.workspaceRepo.findByTaskId("t1")?.linkedBranch).toBeNull();
	});

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