Fix stale Tailtest mtime queueing

[BananaAccurate]

Summary

• require Git dirty/untracked evidence before mtime sweeps queue files in Git worktrees
• preserve legacy-file vs new-file status in Stop and PostToolUse queue entries
• render queued file metadata as bounded untrusted JSON in hook prompts
• add regression coverage for clean tracked mtime churn, dirty tracked files, and untracked files

Tests

• python -m pytest tests/test_mtime_sweep.py tests/test_stop_hook.py tests/test_post_tool_use.py -q — 95 passed
• python -m pytest -q — 410 passed

[BananaAccurate]

@avansaber This is ready for maintainer review and merge. Current state: PR is clean/mergeable at 4e84d5a; no GitHub checks are reported because the upstream repo currently has no visible Actions workflows. Local verification from the Tailtest source: python -m pytest -q passed 443 tests, python -m ruff check . passed, and git diff --check passed. python -m ruff format --check . still reports seven files that need formatting/line-ending normalization before final release packaging, so I am keeping that scope explicit rather than overstating release readiness.