chore: fix sync FS calls in async functions and add debug logging

[avoidwork]

Description

Replace synchronous file system calls (existsSync) in async functions with promise-based equivalents (fs.promises.access) to prevent event loop blocking. Add debug-level logging to the TUI streaming callback catch block for improved observability.

Type of Change

• Bugfix (non-breaking change which fixes an issue)
• Refactor (no functional changes)

Testing

All existing tests pass (1043 tests, 0 failures). No new tests required — this is a behavioral-preserving refactor.

Coverage

• Line coverage maintained (87.40%)

Checklist

• npm run lint passes
• Tests pass with maintained line coverage
• No forbidden patterns used
• Conventional Commit style applied

Related

Fixes #621

[avoidwork]

PR Audit Results

Change: fix-sync-fs-in-async-functions

PR: #622

Audit Summary

Status: ✅ PASS — All requirements met

1. Goal Fulfillment

Goal	Status	Notes
Replace existsSync() in scheduler.js async runNow()	✅ Complete	Replaced with fs.promises.access() + constants.F_OK
Replace existsSync() in cron.js async findSkillScript()	✅ Complete	Replaced with fs.promises.access() + constants.F_OK
Add debug logging to TUI streaming callback	✅ Complete	Added logger.debug() in catch block at line 779

2. Spec Compliance

The implementation matches the spec requirements:

• REQ-1: No sync FS in async functions — ✅ Both locations fixed
• REQ-2: Error observability in streaming callbacks — ✅ Debug logging added
• REQ-3: Backward compatibility — ✅ No API changes, all tests pass

3. Task Completion

All tasks in tasks.md are marked complete:

• scheduler.js fix
• cron.js fix
• TUI debug logging
• Tests pass (1043/1043)
• Lint passes (0 warnings, 0 errors)
• Coverage maintained (87.40%)

4. Quality Check

Files changed: 9 files (3 source, 5 OpenSpec archive, 1 coverage)

Source changes:

• src/scheduler/scheduler.js — Replaced existsSync() with access() in async context
• src/tools/cron.js — Replaced existsSync() with access() in async context
• src/tui/app.js — Added debug logging to streaming callback catch

No issues found:

• No blocking I/O in async contexts
• No silent catch blocks
• No API changes
• All tests pass
• Lint clean
• Coverage maintained

5. OpenSpec Compliance

• ✅ Proposal created and archived
• ✅ Design document created
• ✅ Spec file created with delta requirements
• ✅ Tasks tracked and completed
• ✅ Archive created at openspec/changes/archive/2026-07-25-fix-sync-fs-in-async-functions/

Conclusion

This PR successfully addresses all findings from issue #621. The implementation is clean, well-tested, and follows all project conventions. Ready to merge.