feat: dual-tokenizer search (natural language + code-preserving)

[salishforge]

Summary

Add a code-preserving search mode alongside natural language FTS for agents that work with code.

Design

• Add a `content_code_tsv` TSVECTOR column using 'simple' config (no stemming, preserves symbols)
• Query mode 'code' uses the simple tokenizer
• Existing 'keyword' mode continues using English stemming
• Auto-detect: if query contains code patterns (camelCase, dots, underscores), prefer code tokenizer

Inspiration

Inspired by CCRider (MIT) dual FTS5 tokenizer approach — natural language (Porter stemming) + code-specific (no stemming, preserving symbols).

Closes #55