feat: code index + adaptive breadcrumbs foundation

Code index (sol_code):
- SymbolDocument: file_path, repo_name, language, symbol_name, symbol_kind,
  signature, docstring, branch, source, embedding (768-dim knn_vector)
- CodeIndexer: batch symbol indexer with idempotent upserts
- Branch-aware: symbols scoped to branch with mainline fallback

Breadcrumbs:
- build_breadcrumbs(): adaptive context injection for coding prompts
- Default: project outline via aggregation (modules, types, fns)
- Adaptive: hybrid search (_analyze → symbol matching → BM25 + neural)
- Token budget enforcement with priority (outline first, then relevance)
- format_symbol(): signature + first-line docstring + file:line

Query optimization: uses _analyze API to extract key terms from
free-form user text, matches against actual symbol names in the index
before running the hybrid search.

src/main.rs

@@ -2,6 +2,8 @@ mod agent_ux;
 mod agents;
 mod archive;
 mod brain;
+mod breadcrumbs;
+mod code_index;
 mod config;
 mod context;
 mod conversations;