Add watch paths so integrations refresh on file changes

[shayan-ys]

📝 Summary

This PR adds a per-integration watch option. devdash re-runs a watched integration soon after anything under its watched files or directories changes, so a section such as prompt-tutor's updates within about half a second instead of on the next interval. A watched integration without its own interval stops polling on the global interval.

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🛠️ Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• ⚡ Performance / Refactor (code optimization without behavior changes)
• 📚 Documentation update

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🔍 Changes Made

• watch = ["~/.local/share/prompt-tutor", "~/.config/prompt-tutor"]: a list of paths, validated strictly, with ~ and environment variables expanded. Directories are watched recursively, and a missing path is watched until it is created.
• Detection uses only the standard library: each pass takes a stat snapshot of every watched path. The configured roots follow symlinks; symlinks inside them are not followed, so a link cannot cause a loop. Each pass waits at least 0.25 s, or four times the previous pass's cost, so a large tree cannot keep a core busy.
• Debounce: run once writes are quiet for 0.2 s, or 2 s after the first change in a write burst that never pauses. Runs go through the existing per-integration queue, so a change during a run queues one follow-up.
• Scheduling: with watch and no interval, the integration runs at start, on changes, on its keys, and on r. An explicit interval still applies alongside watching.
• Heading: fetched 3s ago · watching for a watched integration without an interval. --once does not watch.
• Docs: the README integration section and the prompt-tutor example, config.example.toml, and the CHANGELOG.

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🧪 How to Test

1. git checkout feat/integration-watch && uv sync
2. uv run pytest -q && uv run ruff check .
3. Add watch = ["/some/dir"] to an integration that lists that directory, then run devdash. Create a file in the directory: the section shows it within about half a second, and its heading says watching.

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✅ Review and verification

• Review path: material, agent code review; omitted optional reviews: none. CodeRabbit is not active on this repository.
• Review fixes:
  • watch combined with an explicit interval turned watching off.
  • A symlinked watch root never saw changes.
  • Each pass now waits longer when the previous one was slow.
  • A 2 s cap stops a write burst that never pauses from blocking the refresh. The cap is checked on every pass, so it still applies while writes continue.
  • The watched-run test now allows up to 5 s per run.
• Verification:
  • uv run pytest -q: 115 passed; uv run ruff check .: clean.
  • The new test for writes that never pause fails on the earlier elif version and passes now.
  • Terminal smoke with a temporary directory and a stub gh: create and delete events reached the screen in about 0.5 s, the heading said watching, and q exited.
  • The real devdash with the real prompt-tutor re-ran within 0.43 s after its config file was touched.
• Declined findings: none.
• Remaining non-blocking findings: none.