fix(agent): reject non-digit compact dates in chat history

[cursor]

Summary

• chat_history / chat_plan crash the agent process when the LLM passes a localized 8-byte date such as 2026年9 or 2026-9月.
• normalize_date treated any 8-byte string as YYYYMMDD and sliced at byte indexes 4/6, which is mid-character for those CJK forms. There is no catch_unwind on the builtin tool path.
• Fix: only compact 8 ASCII digits are rewritten as YYYY-MM-DD. Other values pass through and miss the file instead of panicking.

Task Linkage

• Task ID: TASK-2026-071
• Task folder: tasks/TASK-2026-071-utf8-chat-date-normalize/

Injected Specs

• spec/verification-integrity.md
• spec/task-artifact-language.md
• spec/rust-conventions.md
• spec/testing-policy.md
• spec/architecture-boundaries.md (if architecture/layering changed)
• spec/security-nonnegotiables.md (if sandbox/security changed)
• spec/docs-sync.md (if behavior/docs/env/commands changed)

Validation Evidence

• Commands executed:
  • cargo test -p skilllite-agent --lib extensions::builtin::chat_data::tests (before fix): 1 passed, 3 failed with byte index 6 is not a char boundary inside 年 / 月
  • Same command after fix: 4 passed
  • cargo test -p skilllite-agent: 251 passed
  • cargo fmt --check: clean
  • cargo clippy --all-targets -- -D warnings: clean
  • cargo test: all workspace packages ok, 0 failed
  • python3 scripts/validate_tasks.py: passed (71 task directories)
• Key results:
  • 20260902 / 2026-09-02 still normalize to 2026-09-02
  • 2026年9 and 2026-9月 no longer panic; tools return Ok

Regression Scope

• Areas likely affected:
  • chat_history and chat_plan date-to-filename lookup
• Explicit non-goals:
  • session_key path sanitization (fix(chat): reject path-escaping session keys #126)
  • other UTF-8 panic drafts (Fix UTF-8 panic in agent planning error paths #92–Fix UTF-8 crash in evolution status reasons #96)
  • recovered write / grep secrets / skill-install symlink drafts

Docs Sync (EN/ZH)

• Not needed
• Tool schema already documents YYYY-MM-DD or YYYYMMDD.

Review Checklist

• Acceptance criteria in tasks/TASK-2026-071-utf8-chat-date-normalize/TASK.md satisfied
• STATUS.md updated with latest progress
• REVIEW.md includes merge readiness decision
• tasks/board.md status is up to date