fix(codex): improve reused session adoption

[ROOOO]

Summary

• allow Codex session scanning to adopt a reused session file when matching activity appears after startup
• keep that fallback limited to activity that still matches the requested cwd
• refuse reused-session adoption when the first fresh matching activity is ambiguous across multiple sessions
• add regression tests for both the successful and ambiguous reuse cases

Testing

• bun x vitest run src/codex/utils/codexSessionScanner.test.ts
• bun run typecheck

[github-actions]

Findings

• None.

Summary
Review mode: initial. No issues found in diff. Residual risk: timing-dependent session scan behavior; coverage limited to unit tests in cli/src/codex/utils/codexSessionScanner.test.ts.

Testing

• Not run (automation)