feat(ingestion): implement brain_query MCP tool

Wraps the existing search.Query function. Same BM25 over
brain/knowledge/ and brain/wiki/ that the HTTP /query endpoint serves.
Plan note: handleCall switch replaces the single-line stub from Task 1
— no unknownToolError type to remove since Task 1 inlined the error.

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

ingestion/internal/mcp/handlers.go

@@ -1,6 +1,12 @@
 package mcp
 
-import "encoding/json"
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/search"
+)
 
 // tools returns the tool descriptors. Handler bodies for each tool are filled
 // in subsequent tasks; this file currently only provides the descriptors.
@@ -73,3 +79,26 @@ func (s *Server) tools() []map[string]any {
 		},
 	}
 }
+
+type brainQueryArgs struct {
+	Query string `json:"query"`
+	Limit int    `json:"limit,omitempty"`
+}
+
+func (s *Server) brainQuery(ctx context.Context, args json.RawMessage) (json.RawMessage, error) {
+	var a brainQueryArgs
+	if err := json.Unmarshal(args, &a); err != nil {
+		return nil, fmt.Errorf("parse args: %w", err)
+	}
+	if a.Query == "" {
+		return nil, fmt.Errorf("query is required")
+	}
+	if a.Limit == 0 {
+		a.Limit = 5
+	}
+	results, err := search.Query(s.brainDir, a.Query, a.Limit)
+	if err != nil {
+		return nil, fmt.Errorf("search: %w", err)
+	}
+	return json.Marshal(map[string]any{"results": results})
+}