fix(diff-filter): ignore Anchor generated directory

[tolga-tom-nook]

Summary

• Adds .anchor/ to the generated directory prefixes so Anchor build/test artifacts are ignored by the diff filter.
• Covers both nested and top-level .anchor/ paths in classifyPath tests.

Validation

• pnpm --filter @ghbounty/diff-filter test -- --run tests/classify.test.ts → 53 passed
• pnpm --filter @ghbounty/diff-filter test → 70 passed
• Commit hook ran full workspace checks:
  • pnpm -r typecheck passed
  • pnpm -r test passed

Fixes #67

[vercel]

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