Dev watcher triggers rebuilds for .gitignore'd directories

[astrobot-houston]

Originally reported by @noahkhomer18 in #325.

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Describe the Bug

The flue dev file watcher does not consult the project's .gitignore when deciding which file changes should trigger a rebuild. Directories that are listed in .gitignore — such as evals/results/ or custom tool output folders — still cause unnecessary rebuild churn when their contents change.

Expected Behavior

Paths matching .gitignore patterns should be excluded from the dev watcher so that changes to generated or ignored directories do not trigger rebuilds.

Steps to Reproduce

1. Add a directory to .gitignore (e.g., evals/results/).
2. Run flue dev to start the dev watcher.
3. Write or modify a file inside that ignored directory (e.g., create evals/results/output.json).
4. Observe that the watcher picks up the change and triggers an unnecessary rebuild.

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Original implementation from #325 by @noahkhomer18