Respect .gitignore negation patterns during indexing

[caioribeiroclw-pixel]

Fixes #397.

This makes file-based ignore matching honor ! negation patterns instead of treating them as inert literal patterns. The bug in #397 can silently remove intentionally tracked/whitelisted directories from the index, which is especially risky for code-search agents because the user sees a completed index but the search universe is incomplete.

What changed:

• Context.matchesIgnorePattern now evaluates ignore rules in order and lets later !pattern rules re-include matching paths.
• FileSynchronizer.shouldIgnore uses the same ordered negation behavior so initial indexing and sync hashing stay aligned.
• Added regression coverage for the reported WordPress-style pattern:

  wp-content/plugins/*
  !wp-content/plugins/app
  !wp-content/plugins/backoffice

  covering both initial indexing and sync hashing.

Validation:

• git diff --check
• from packages/core: ./node_modules/.bin/jest src/context.ignore-patterns.test.ts --runInBand
• from packages/core: ./node_modules/.bin/tsc --noEmit

Note: I intentionally kept this narrow and did not replace the matcher with a full gitignore parser. The goal is to fix the silent exclusion case from #397 without changing unrelated ignore semantics.