chore(brain): de-surface brain_index from the model-facing toolset (#78)

brain_index is auto-called after every write, and its own description
said so. A verb surfaced to callers that callers should never call is
cognitive load with no decision behind it — ADR-0012 rule 4.

Removed from tools/list only. The dispatch case stays, so maintenance
can still rebuild a Map of Content by hand: de-surfacing hides the verb,
not the operation. A test pins both halves, since removing the descriptor
and removing the capability look identical from the tool list.

Nothing called it directly — grepped the skills, specs and docs.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BeAp5LTscnz2W7Ubt4eJ6m

ingestion/internal/mcp/server_test.go

@@ -59,11 +59,35 @@ func TestServerToolsList(t *testing.T) {
 	assert.ElementsMatch(t, []string{
 		"brain_query", "brain_write", "brain_update", "brain_get",
 		"brain_pending", "brain_promote",
-		"brain_index", "brain_tunnel",
+		"brain_tunnel",
 		"brain_ingest_raw", "brain_ingest",
 		"brain_answer", "brain_classify", "brain_graph", "brain_context",
 		"session_log",
 	}, names)
+
+	// brain_index is auto-called after every brain_write, so surfacing it as
+	// a caller verb is cognitive load with no decision behind it (ADR-0012
+	// rule 4, #78). The capability stays reachable — see the admin test below.
+	assert.NotContains(t, names, "brain_index")
+}
+
+func TestBrainIndexStaysCallableForAdminUseAfterDeSurfacing(t *testing.T) {
+	// De-surfacing hides the verb from the model-facing list; it must not
+	// remove the operation. A maintenance caller still needs to rebuild a
+	// Map of Content by hand.
+	srv := mcp.NewServer(t.TempDir(), nil, nil, nil)
+
+	req := httptest.NewRequest(http.MethodPost, "/mcp", body(t, map[string]any{
+		"jsonrpc": "2.0", "id": 3, "method": "tools/call",
+		"params": map[string]any{"name": "brain_index", "arguments": map[string]any{}},
+	}))
+	rr := httptest.NewRecorder()
+	srv.ServeHTTP(rr, req)
+
+	require.Equal(t, http.StatusOK, rr.Code)
+	var resp map[string]any
+	require.NoError(t, json.Unmarshal(rr.Body.Bytes(), &resp))
+	assert.NotContains(t, resp, "error", "brain_index must still dispatch: %v", resp["error"])
 }
 
 func TestServerNotificationGetsNoBody(t *testing.T) {