refactor(tools): remove zeph-index dependency, inline language detection

[bug-ops]

Summary

zeph-tools depends on zeph-index only for detect_language in search_code.rs. Both are Layer 1 crates — this is a same-layer violation.

Fix: inline language detection and grammar/query setup directly in search_code.rs using the same tree-sitter crates already available in zeph-tools.

Acceptance criteria

• zeph-tools/Cargo.toml does not list zeph-index
• All existing structural search tests pass
• cargo clippy --workspace --features full -- -D warnings passes

Part of EPIC-01 (issue 01-05)