[JacobStephens2]

Summary

Worktree independently acquires Issue branches without replacing local-only work. Issue branch selection uses Worktree's rule for advisory preflight before the Claim.

acquire fresh / Continuation
  hold the worktree lock
  fetch origin and pin its remote-tracking commit
  validate the sampled local Issue branch head
  if equal Continuation: attach the existing branch
  otherwise: create without force at the pinned commit

Closes #485

Evidence

• Before: continuation_refuses_local_only_work_without_selection_and_preserves_its_head failed: acquisition replaced the saved local-only SHA with origin's older SHA.
  After: the regression passes; acquisition refuses and retains the exact local and origin heads without adding a worktree.

• Before: a local branch named origin/issue-7 caused acquisition to compare against the wrong head.
  After: acquisition pins fully qualified remote-tracking refs; the shadowing regression passes for fresh and continued checkouts and retains the shadowing branches.

• Real-Git tests cover post-preflight local commits and origin advancement, unchanged reflogs for equal branches, absent branches, occupied paths and branches already used by another worktree. CLI coverage verifies refusal before a session or Delivery and ordinary Claim behavior.

• Focused Worktree, Issue branch selection, Continuation, fresh and numbered Run tests and Spec run Ticket/Claim checks passed. cargo fmt --check and cargo clippy --all-targets -- -D warnings passed. cargo test -- --test-threads=1 passed: 1322 tests passed, 1 ignored across 39 test binaries.

• Two normal cargo test attempts stopped in unchanged tests: child-run script startup reported Text file busy, and a Harness catalog interruption assertion failed. Both tests passed in isolation; the complete serial run passed with cargo test -- --test-threads=1.

Test seams

• Worktree acquisition (create_fresh and continue_existing) and its shared advisory preflight against real Git, a temporary Launch directory and a bare local origin; observe refusal, exact refs, worktree registration and filesystem preservation.
• Issue branch selection's existing interface for naming and advisory refusal.
• The thirdshift CLI against real Git and fake GitHub/Harness adapters for early and late refusal, Claim behavior, fresh/numbered/continued Runs and Spec run Ticket checkout.

Merge Danger

Door: two-way

The change is reversible and adds no persistent format or migration.

Blast Radius: acquisition

Runs and Spec runs refuse late local mismatches before sessions start. Native Git add failures can leave a newly created local branch; pre-existing refs and occupied paths are preserved.

Unaddressed findings

Standards

None. The parallel review and final follow-up reported no findings.

Spec

None. The parallel review and final follow-up reported no findings.

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