[WRONG BRANCH] fix(codex): preserve concurrent obsolete shim replacement

[luvs01]

Motivation

• Obsolete Unix shim rollback could delete a concurrently installed wrapper because it fell back to a public revision marker when a transaction had no recorded fingerprint.
• The public marker is not proof of ownership and allowed a race where rollback removed another updater's shim, potentially breaking the user's codex launcher.

Description

• Remove the marker-text fallback in rollbackObsoleteUnixShimRefresh() so rollback only unlinks a wrapper when the transaction recorded a fingerprint and the current file still matches it.
• Add a test-only race hook setCodexShimObsoleteWriteHookForTests() to exercise the post-write replacement window.
• Invoke the new hook immediately after the obsolete-path writeShim() so tests can simulate a concurrent replacement.
• Add a regression test obsolete Unix shim rollback preserves a concurrent current shim in tests/codex-shim.test.ts that verifies the rollback preserves a concurrent replacement, the backup, and the recorded state.

Testing

• Ran the focused regression: bun test tests/codex-shim.test.ts --test-name-pattern 'obsolete Unix shim rollback preserves' and it passed.
• Ran the full shim test file: bun test tests/codex-shim.test.ts, which showed the new regression passing but exposed two pre-existing timing-sensitive failures unrelated to this change.
• Ran static checks: bun run typecheck and bun run privacy:scan, both succeeded.
• Attempted the full suite with bun run test; the run produced many passing tests but also encountered unrelated timeouts in other subsystems under the long test load (these failures are not caused by this patch).

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Codex Task

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📥 Commits

Reviewing files that changed from the base of the PR and between 1193075 and 95d5032.

📒 Files selected for processing (2)

• src/codex/shim.ts
• tests/codex-shim.test.ts

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[github-actions]

✅ Deterministic PR hygiene checks passed.

[github-actions]

⏳ DRAFT

• wrong target branch (main); retarget to dev.

What to do

• Retarget this PR to dev — all contributions go to dev.

Its title has been prefixed with [WRONG BRANCH].
This pull request was already a draft. Its draft status will be preserved after every issue above is resolved.