Retry contended path claims during branchless attachment

[bb-slop-cop]

🚨 SLOP COP 🚨 · new-issue-autopilot

Human comments

What was wrong

Branchless checkout attachment failed immediately when another preparation held the path claim. That claim remains held until the first thread finishes attaching. Reproduction report.

What changed

Retry branchless claims every 50 ms for up to the existing 15-minute attachment timeout, honoring cancellation. Preserve exclusive claims and immediate refusal of competing branch changes. Report preparation contention accurately.

The environment provisioning change totals 171 text lines (163 additions, 8 deletions) across 3 files. No dependency, generated file, stored-data, public API, or protocol changes.

How you verified

• The focused real-SQLite regression failed on trusted main in two clean checkouts: expected ready, received error. It passes after the fix and verifies reuse of the same environment.
• Turbo server environment orchestration tests: 65 passed.
• Turbo checkout-plugin tests: 34 passed, including cancellation, timeout, and competing branch refusal.
• Turbo server and plugin typechecks, plugin bundle preparation, and git diff --check passed.
• Live provider turns and CLI failure rates were not tested.

Fixes #3609

AGENT GENERATED