feat([issue-5610]): add evidence-led module hygiene audits

[atomantic]

Summary

• add an on-demand module-hygiene scheduled audit with file-issues as the safe default
• make the audit evidence-led, bounded, prior-work-aware, and explicit about reusable-surface discoverability
• force remediation runs into isolated worktrees while keeping file-issues runs read-only
• preload open issues and pull requests for cross-task deduplication
• preserve task-type skill routing when users customize the scheduled prompt

Validation

• server test suite: 36,793 passed, 23 skipped
• client test suite: 10,620 passed
• client lint: clean
• client production build: clean
• focused post-review regression: 250 passed
• required local code review: clean after one routing fix

Closes #5610