fix(core): honor negated ignore patterns

[LiJzd]

Summary

• support negated ignore patterns by applying ignore rules in order and letting the last matching rule win
• apply the same behavior in initial indexing and file synchronization
• add regression coverage for .gitignore patterns like wp-content/plugins/* followed by !wp-content/plugins/app

Closes #397

Validation

• git diff --check
• npx pnpm@10 install --frozen-lockfile
• npx pnpm@10 --filter @zilliz/claude-context-core test -- context.ignore-patterns.test.ts --runInBand
• npx pnpm@10 --filter @zilliz/claude-context-core typecheck
• npx pnpm@10 --filter @zilliz/claude-context-core build

Notes

• npx pnpm@10 --filter @zilliz/claude-context-core lint currently exits before linting because ESLint 9 cannot find an eslint.config.(js|mjs|cjs) file while the repo has .eslintrc.js.
• Local Node is v25.2.1 while the repo declares Node >=20 and <24; the targeted tests, typecheck, and build still passed in this environment.