feat(search): the agent's search can ask for every kind the corpus has (#4250)

The engine took `note_type` and `task_kind` all along. What was missing was a
way to say them: the MCP tool's `content_type` knew `note`, `task` and `all`,
and `/api/search` knew the same two — so an agent could not ask "has this
snippet already been recorded" or "what lessons apply here" without searching
everything and reading past the rest. Browse offered nine kinds from the same
data.

The cause is that each door kept its own map. `_FACETS` in services/knowledge
is where a kind is declared, and #3161 made adding one a single edit by
generating the SQL filter, the Python predicate and the door's validation from
it — but the two search doors were written before that and never joined. So
this adds the third dialect, `search_filters_for`, and one composition over it,
`content_type_filters`, and both doors now derive instead of listing.

Two names keep a meaning of their own, and the docstrings say so: `all` is no
filter, and `note` is BROAD — any non-task, snippets and lessons included —
where the browse facet of the same name is narrow (`note_type == 'note'`).
They are left different deliberately; narrowing this one would stop returning
snippets to every caller that already asks this way.

An unrecognised kind is now refused rather than answered. Both doors used to
fall through: the MCP tool into a filter matching no row, the route into no
filter at all, so `?content_type=snippets` returned the whole corpus while
looking like a narrowed search. An empty result set is a claim — "the corpus
holds nothing like this" — and an agent acts on that claim by building the
thing it could not find, so a typo must not be able to make it.

The docstring is the agent-facing contract (#2846), and a test now holds it to
the table: every kind `_FACETS` declares has to appear in it, because a filter
an agent has not been told about is unreachable however well it is wired.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

src/scribe/routes/search.py

@@ -8,6 +8,7 @@ from scribe.services.embeddings import (
     INTERACTIVE_SEARCH_THRESHOLD as _REST_SEARCH_THRESHOLD,
 )
 from scribe.services.embeddings import semantic_search_notes
+from scribe.services.knowledge import content_type_filters
 from scribe.services.retrieval_telemetry import record_retrieval
 
 # The interactive floor lives in embeddings.py now, shared with Browse search
@@ -16,15 +17,6 @@ from scribe.services.retrieval_telemetry import record_retrieval
 search_bp = Blueprint("search", __name__, url_prefix="/api/search")
 
 
-def _content_type_to_is_task(content_type: str) -> bool | None:
-    """Map content_type query param to semantic_search_notes is_task arg."""
-    if content_type == "note":
-        return False
-    if content_type == "task":
-        return True
-    return None  # "all" or unknown → no filter
-
-
 @search_bp.route("", methods=["GET"])
 @login_required
 async def search_route():
@@ -33,9 +25,18 @@ async def search_route():
     if not q:
         return jsonify({"error": "q is required"}), 400
 
-    content_type = request.args.get("content_type", "all")
     limit = min(request.args.get("limit", 10, type=int), 50)
-    is_task = _content_type_to_is_task(content_type)
+    # Every kind the facet table declares, derived rather than mapped here —
+    # this route used to know exactly two and read anything else as "no
+    # filter", so `?content_type=snippets` silently returned the whole corpus
+    # (#4250). An unknown kind is now a 400 naming the ones that exist: a
+    # result set is an answer, and it should not be able to answer a question
+    # nobody asked.
+    try:
+        filters = content_type_filters(request.args.get("content_type", "all"))
+    except ValueError as exc:
+        return jsonify({"error": str(exc)}), 400
+    is_task = filters.get("is_task")
     # Same association filters the MCP tool takes (#33). Optional, default
     # global: this route has NO frontend consumer today (measured 2026-08-08 —
     # the web UI searches through /api/knowledge), so it serves API callers,
@@ -46,7 +47,7 @@ async def search_route():
     t0 = time.perf_counter()
     report: dict = {}
     results = await semantic_search_notes(
-        uid, q, limit=limit, is_task=is_task, threshold=_REST_SEARCH_THRESHOLD,
+        uid, q, limit=limit, **filters, threshold=_REST_SEARCH_THRESHOLD,
         project_id=project_id, system_id=system_id,
         # The user typed this, so it reaches everything they may read.
         scope="read",