fix(server): follow placeholder branches after checkout updates

[Yash-Singh1]

If you run out of usage for your branch generation model, the placeholder branch can stay for longer.

What Changed

• Follow a real checked-out branch when a thread still records a saved placeholder branch.
• Preserve the existing safety check for worktrees shared by another thread.
• Refresh the associated pull request after adopting the branch.
• Added regression coverage for placeholder-branch adoption and shared-worktree protection.

Why

Threads can retain a temporary placeholder branch name after automatic branch naming completes. When checkout updates the worktree, the checkpoint reactor should adopt the real branch and refresh its pull request, while avoiding adoption when the worktree is shared.

Checklist

• This PR is small and focused
• I explained what changed and why

[macroscopeapp]

Approvability

Verdict: Would Approve

Macroscope's review found this PR approvable — This is a narrowly scoped server bug fix that follows real branches after placeholder checkout updates while preserving detached-checkout and shared-worktree protections. Regression tests cover branch adoption, pull-request refresh, and the safety cases.

Not approved because:

• Monthly spending limit reached (workspace setting). Approvability relies on correctness review in order to determine eligibility

^{Review your spending limits in Billing settings. You can add or adjust custom eligibility rules. Learn more.}

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.6 KiB	13.6 KiB	−17 B (−0.1%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.0 KiB	7.0 KiB	0 B (0.0%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.6 KiB	6.5 KiB	−17 B (−0.3%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	57.1 KiB	57.1 KiB	0 B (0.0%)	66.4 KiB	✅
Codex	Live turn messages	10	10	0 (0.0%)	21	✅
Claude	Total thread wire	13.6 KiB	13.6 KiB	+38 B (+0.3%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.0 KiB	7.0 KiB	−1 B (−0.0%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.5 KiB	6.6 KiB	+39 B (+0.6%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	57.8 KiB	57.9 KiB	+44 B (+0.1%)	66.4 KiB	✅
Claude	Live turn messages	9	10	+1 (+11.1%)	21	✅

Baseline: efeac14 · PR result: a68c63a · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 113.9 KiB
• Claude decoded thread snapshot: 114.6 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.