feat: add rtk rgai command for semantic code search

[heAdz0r]

Summary

Extracted from #118 per reviewer feedback. This is the actual feature implementation — the command that #118's docs/hooks referenced but didn't include.

rtk rgai is a Rust-native semantic search that scores files by term relevance without requiring external embedding services.

Usage

rtk rgai "auth token refresh"              # multi-word semantic query
rtk rgai auth token refresh --compact      # unquoted, compact output
rtk rgai "error handler" --json            # machine-readable JSON
rtk rgai "database migration" -t rust      # filter by file type
rtk rgai "auth flow" -p ./src              # explicit path

How it works

1. Parse query → remove stop words, stem tokens
2. Walk project (gitignore-aware, skip binary/large files)
3. Score each line: term match (+1.4-1.7), phrase match (+6.0), multi-term bonus (+1.2), symbol definition boost (+2.5), comment penalty (×0.7)
4. Score each file: path relevance + top snippet scores + match density
5. Rank files, emit top N with context snippets

Token savings

• 75-90% reduction vs raw grep output (fewer files, ranked by relevance, truncated lines)
• Compact mode: 1 snippet per file, 0 context lines
• JSON mode: structured output for programmatic consumption

Changes

• src/rgai_cmd.rs: 789 lines — full search implementation with scoring, ranking, output formatting
• src/main.rs: Commands::Rgai variant, match arm, normalize_rgai_args() for backward-compat path detection

Test plan

• cargo test rgai — 8 tests pass (5 unit + 3 arg normalization)
• cargo test — 321 total tests pass
• cargo fmt --all --check — clean
• cargo clippy --all-targets — no new warnings
• Manual: rtk rgai "token tracking" -p . returns ranked results from this repo