feat: add semantic search tool for code symbol indexing and searching

- Implemented a new tool for semantic code search that indexes Rust code symbols (functions, structs, enums, traits, modules) and allows searching by name, concept, or meaning.
- Introduced a symbol index structure with methods for rebuilding the index and searching symbols.
- Added regex patterns for extracting various code symbols from Rust source files.
- Implemented scoring logic for search results based on exact matches, prefix matches, and context relevance.
- Created a web search tool that interacts with a SearXNG instance to fetch documentation and API information based on user queries.
- Added a diff preview overlay for rendering git diff output with color-coded additions and deletions in a TUI interface.

apps/infrastructure/src/subagent/auto/mod.rs

@@ -1,4 +1,5 @@
 //! Auto-subagents — automatically run review/test subagents at the end of
 //! each turn.
 
+pub mod engine;
 pub mod paths;