docs(fb-38): wire schema + README + design + SKILL + INDEX

integrations/claude-code/kebab/SKILL.md

@@ -55,6 +55,7 @@ Input:
 - **`max_tokens` / `snippet_chars` / `cursor` (p9-fb-34)** — agent budget controls. Set `max_tokens` to cap result wire size (chars/4 estimate); set `cursor` to the previous response's `next_cursor` to fetch the next page.
 - **p9-fb-36 filter inputs:** `tags` (string array — OR-within, AND across keys), `lang` (BCP-47 language code), `path_glob` (glob pattern matched against doc path), `trust_min` (`"primary"` | `"secondary"` | `"generated"` — includes that level and above), `media` (string array — IN-list of `"markdown"` | `"pdf"` | `"image"` | `"audio"` | `"other"`; alias `"md"` → `"markdown"`), `ingested_after` (RFC3339 UTC string), `doc_id` (exact doc UUID). AND combinator across keys. Invalid `ingested_after` or unknown `trust_min` → `error.v1.code = invalid_input`. Unknown `media` value → empty hits, no error.
 - Output is `search_response.v1`: `{ hits: search_hit.v1[], next_cursor: string|null, truncated: bool }`. Iterate `response.hits[]` for individual hits. Key hit fields: `rank`, `score`, `doc_path`, `heading_path[]`, `section_label`, `snippet`, `citation` (line range / page), `chunk_id`.
+- **`hits[].score_kind` (p9-fb-38):** `"rrf"` (hybrid) / `"bm25"` (lexical) / `"cosine"` (vector). Declares the meaning of the top-level `score` — it is a **ranking signal**, not a confidence value. If you need a trust threshold, use `retrieval.lexical_score` (BM25 raw) / `retrieval.vector_score` (cosine raw) instead of the top-level `score`.
 - Cite back to the user as `doc_path § heading_path[-1]` so they can open the source.
 - When `truncated: true`, the budget loop modified the page (snippet shortening or k reduction). `next_cursor` is **independent** — non-null whenever more hits may be reachable. Caller may widen `max_tokens` (re-issue same query for fuller snippets / more hits per page) or follow `next_cursor` (advance through more hits) or both. Mismatched cursor (corpus_revision changed) returns `error.v1.code = stale_cursor` — re-issue the search to obtain a fresh one.
 - **`trace: true` (p9-fb-37)** — debug aid. Response carries an extra `trace` block: `lexical[]` + `vector[]` (pre-fusion candidates), `rrf_inputs[]` (RRF union before final cut), and `timing` (`lexical_ms`, `vector_ms`, `fusion_ms`, `total_ms`). Trace bypasses the search cache (always cold). Use sparingly — it bloats the wire response and is for diagnosing "why did this hit / not hit", not normal retrieval.