feat(search): Add hybrid search engine with FTS5, vector, and RRF fusion

Implements the search module providing three search modes:

- Lexical (FTS5): Full-text search using SQLite FTS5 with safe query
  sanitization. User queries are automatically tokenized and wrapped
  in proper FTS5 syntax. Supports a "raw" mode for power users who
  want direct FTS5 query syntax (NEAR, column filters, etc.).

- Semantic (vector): Embeds the search query via Ollama, then performs
  cosine similarity search against stored document embeddings. Results
  are deduplicated by doc_id since documents may have multiple chunks.

- Hybrid (default): Executes both lexical and semantic searches in
  parallel, then fuses results using Reciprocal Rank Fusion (RRF) with
  k=60. This avoids the complexity of score normalization while
  producing high-quality merged rankings. Gracefully degrades to
  lexical-only when embeddings are unavailable.

Additional components:

- search::filters: Post-retrieval filtering by source_type, author,
  project, labels (AND logic), file path prefix, created_after, and
  updated_after. Date filters accept relative formats (7d, 2w) and
  ISO dates.

- search::rrf: Reciprocal Rank Fusion implementation with configurable
  k parameter and optional explain mode that annotates each result
  with its component ranks and fusion score breakdown.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/search/mod.rs

@@ -0,0 +1,14 @@
+mod filters;
+mod fts;
+mod hybrid;
+mod rrf;
+mod vector;
+
+pub use fts::{
+    generate_fallback_snippet, get_result_snippet, search_fts, to_fts_query, FtsQueryMode,
+    FtsResult,
+};
+pub use filters::{apply_filters, PathFilter, SearchFilters};
+pub use rrf::{rank_rrf, RrfResult};
+pub use vector::{search_vector, VectorResult};
+pub use hybrid::{search_hybrid, HybridResult, SearchMode};