fix(diff-filter): ignore Anchor .anchor/ generated directory

[weilixiong]

Summary

Adds .anchor/ to GENERATED_DIR_PREFIXES so that Anchor framework build artifacts (test ledger, IDL cache) are filtered from the diff shown to Sonnet.

Changes

• packages/diff-filter/src/patterns.ts: Added .anchor/ to GENERATED_DIR_PREFIXES array
• packages/diff-filter/tests/classify.test.ts: Added contracts/solana/.anchor/test-ledger/genesis.bin test case

Verification

# All 69 existing tests pass
cd packages/diff-filter && npx vitest run
#
# ✓ tests/parse.test.ts (8 tests)
# ✓ tests/filter.test.ts (9 tests)
# ✓ tests/classify.test.ts (52 tests)
# Test Files  3 passed (3)
# Tests  69 passed (69)

Closes #67

[vercel]

@weilixiong is attempting to deploy a commit to the weareghbounty-6269's projects Team on Vercel.

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[weilixiong]

Hi maintainers! Just a friendly ping on this PR — it adds .anchor/ to GENERATED_DIR_PREFIXES to prevent git diff from picking up generated directory changes.

All 69 existing tests pass (including the new test for .anchor/). Would appreciate a review when you get a chance. Thanks!