[JacobStephens2]

Summary

Coordinate fetches with Worktree changes in the shared local repository:

common Git directory: thirdshift-worktree-refs.lock
  fetch                → check local connectivity → release
  Worktree acquisition → create or recover        → release
  Worktree disposal    → remove checkout + branch → release

An unchanged-origin Linux reproduction reached the exact Git 2.55.0 CI error. Fetch checks local refs and sibling Worktree heads, so atomic origin deletion does not protect that read. Diagnosis and evidence distinguish observations from the inference about GitHub transport. Ticket sessions continue to run concurrently.

Closes #509

Evidence

• Before: a_sibling_fetch_waits_for_worktree_creation_to_finish failed with bad object worktrees/work-issue-22/HEAD; the disposal regression also detected an overlapping fetch with its guard removed.
  After: both controlled regressions pass on Git 2.43.0 and 2.55.0, alongside checks that missing required refs, missing objects, and transport failures remain errors. No new retries.
• Both reported Spec run scenarios passed 10 times each on each Git version, 40 runs total, retaining their Base fix sharing and maximum-three-Tickets assertions.
• Validation: cargo test (1431 passed; 1 existing ignored test), 22 Git checks, 110 Worktree checks, cargo fmt --check, and cargo clippy --all-targets -- -D warnings.

CI notes

• Linux test failure: child_run::execution_tests::a_pending_collection_waits_for_its_child_and_retires_it_before_delivery hit the pre-existing Text file busy (os error 26) fixture-launch flake tracked in #483; the child-run code is identical to main, the exact test passed on both branches, all local CI checks passed (cargo test: 1,431 passed; 1 existing ignored test), and the Linux rerun passed.

Test seams

• Git::fetch: required-ref/object/transport errors.
• Worktree::create_fresh and scope-exit disposal alongside a sibling Worktree::merge_base_branch: controlled shared-ref transitions.
• Spec run CLI: concurrent Tickets share one Base fix and retain the default maximum of three simultaneous Tickets.

Review

Reviewed with thirdshift-code-review against main (d000eee). Standards finding S1, duplicated test orchestration, was addressed with a shared helper and verified by the reviewer. Spec reported no findings.

Unaddressed findings

Standards

None.

Spec

None.

Merge Danger

Door: two-way

Revert the locking change to restore the previous behavior.

Blast Radius: Git

Fetches and Worktree acquisition/disposal briefly wait for each other within one shared repository.

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