fix(fb-34): address PR #125 round 1 review

- error_wire: StructuredError wrapper preserves ErrorV1 through
  anyhow → classify pipeline. Adds downcast short-circuit so
  cursor::decode's typed code = "stale_cursor" reaches the wire
  instead of being string-formatted to code = "generic".
- app: search_with_opts now wraps cursor::decode error in
  StructuredError instead of anyhow! string format.
- test: error_wire pins both negative (bare anyhow → not
  stale_cursor) AND positive (StructuredError → stale_cursor)
  invariants. CLI integration test runs end-to-end and asserts
  error.v1.code on stderr.
- app: next_cursor only emitted on full-page (k-pop) path; drop
  speculative emit on snippet-only truncation that would point at
  a different page than the agent expected.
- cursor: differentiate malformed-base64 / malformed-payload /
  revision-mismatch error messages; all keep code = stale_cursor.
- test: cursor_rejected fixture uses .expect() to fail loud on
  cursor non-emission instead of silent skip.
- test: max_tokens=0 → 1-hit floor + truncated=true.
- docs: SKILL.md + schema description distinguish snippet-shrink
  (widen) vs k-pop (paginate) truncated cases. HOTFIXES notes
  --no-cache semantic shift (cached path + clear vs uncached path).

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

docs/wire-schema/v1/search_response.schema.json

@@ -9,6 +9,6 @@
     "schema_version": { "const": "search_response.v1" },
     "hits":           { "type": "array", "description": "search_hit.v1[]" },
     "next_cursor":    { "type": ["string", "null"], "description": "Opaque base64 cursor for next page; null when no more hits." },
-    "truncated":      { "type": "boolean", "description": "True when budget forced snippet shortening or k reduction. Caller can request next page via next_cursor or pass higher k." }
+    "truncated":      { "type": "boolean", "description": "True when budget forced snippet shortening or k reduction. When `next_cursor` is also non-null, k was reduced — paginate via cursor. When `next_cursor` is null but `truncated: true`, only snippets were shortened — widen `max_tokens` to get fuller snippets." }
   }
 }