feat(app): SearchResponse.trace + opts.trace threading (fb-37)

Adds the `trace: Option<SearchTrace>` field to `SearchResponse` and
threads `SearchOpts.trace` through `App::search_with_opts`. When the
caller sets `opts.trace = true` the path bypasses the LRU search cache
and runs through `HybridRetriever::search_with_trace`, which dispatches
all 3 SearchModes internally; this means `--trace` requires embeddings
(same constraint as `--mode hybrid`). The non-trace path keeps its
exact prior behavior with `trace: None` stamped on the response.

Picked up Task 1 / Task 3 follow-ups in the same commit so the
workspace compiles: SearchOpts struct-literals in kebab-cli/main.rs +
kebab-mcp/tools/search.rs default the new `trace` field to false, and
the schema-wrapper test in kebab-cli/wire.rs fills the new
media_breakdown / lang_breakdown / index_bytes / stale_doc_count fields
on Stats with `Default::default()`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/kebab-mcp/src/tools/search.rs

@@ -118,6 +118,7 @@ pub fn handle(state: &KebabAppState, input: SearchInput) -> CallToolResult {
         max_tokens: input.max_tokens,
         snippet_chars: input.snippet_chars,
         cursor: input.cursor,
+        trace: false,
     };
     let cfg_clone = (*state.config).clone();
     match kebab_app::search_with_opts_with_config(cfg_clone, query, opts) {