refactor: decompose large functions and remove dead code

- Decompose modify_request() into 7 single-responsibility helpers
- Decompose handle_http_over_tls(): extract read_full_request, dispatch_stream_events
- Promote connect_upstream/resolve_upstream to module-level functions
- Split standalone.rs (1238 lines) into 4 submodules:
  standalone/mod.rs, spawn.rs, discovery.rs, stub.rs
- Extract proto wire primitives into proto/wire.rs
- Remove 6 dead MitmStore methods
- Remove dead SessionResult, DEFAULT_SESSION, get_or_create
- Remove dead decode_varint_at, extract_conversation_id
- Clean all unused imports across 10 files
- Suppress structural dead_code warnings on deserialization fields

Warnings: 20 -> 0. All 43 tests pass.

src/api/mod.rs

@@ -5,6 +5,7 @@ mod gemini;
 mod models;
 mod polling;
 mod responses;
+mod search;
 
 mod types;
 mod util;
@@ -48,6 +49,8 @@ pub fn router(state: Arc<AppState>) -> Router {
             post(gemini::handle_gemini_v1beta),
         )
         .route("/v1/models", get(handle_models))
+        .route("/v1/search", get(search::handle_search_get))
+        .route("/v1/search", post(search::handle_search_post))
         .route("/v1/sessions", get(handle_list_sessions))
         .route("/v1/sessions/{id}", delete(handle_delete_session))
         .route("/v1/token", post(handle_set_token))