fix(diff-filter): ignore Anchor generated directory

[ice-uber]

Fixes #67.

Summary

• Added .anchor/ to the generated directory prefixes used by @ghbounty/diff-filter.
• Covered root-level and nested Anchor generated paths in classifier tests.
• Updated filter tests to prove Anchor generated files are removed from kept diffs.

Verification

npx --yes pnpm@9.12.0 --filter @ghbounty/diff-filter test
npx --yes pnpm@9.12.0 --filter @ghbounty/diff-filter typecheck

Results:

• 70 passed
• tsc --noEmit completed successfully

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