fix: pr-bug-scan validated finding from #4462

[buf0-bot]

Automated fix-PR from pr-bug-scan for parent #4462.

Fixer status: FIXED_WITH_CODE_PROOF
Summary: Race git fetch in refreshBranchCleanupTargetRefs against a 5s timeout so worktree.rm cannot stall on unreachable remotes.
Blocked proof link: src/shared/git-branch-cleanup.ts:61
Typecheck: skipped

Proof (from validator)

• C1 — proof_type: code_analysis
  • trigger: User invokes worktree.rm (mobile or desktop) on a worktree whose branch was squash-merged upstream, while the saved remote is unreachable.
  • observable: removeWorktree promise does not resolve until git fetch errors or completes; on slow/unreachable remotes this stalls the worktree.rm RPC and blocks the calling UI flow.
  • path:
    • src/main/git/worktree.ts:592 removeWorktree calls git branch -d
    • src/main/git/worktree.ts:610 -d fails with 'not fully merged' (squash-merge shape)
    • src/main/git/worktree.ts:613 catch invokes deleteAlreadyMergedBranchAfterSafeDeleteFailure
    • src/main/git/worktree.ts:626 refreshBranchCleanupTargetRefs(runGit, targetRefs) is awaited synchronously
    • src/shared/git-branch-cleanup.ts:61 await readOptionalGitStdout(runGit, ['fetch', '--prune', remote])
    • src/shared/git-branch-cleanup.ts:3 readOptionalGitStdout has no timeout, just try/catch

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Generated by pr-bug-scan (proof-machine architecture). Human approval required before merge.

[pullfrog]

✅ No new issues found.

Reviewed changes — races git fetch --prune in refreshBranchCleanupTargetRefs against a 5s timeout so an unreachable remote cannot stall worktree.rm.

• Add Promise.race timeout to refreshBranchCleanupTargetRefs — the git fetch --prune for refreshing remotes before branch-cleanup now times out at 5s via Promise.race, with the timer cleaned up in a finally block.

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