feat(brain-mcp): audit-log every tools/call with principal and scope

brain_query and brain_read left no record of who read what; only capture
and governance edits reached the audit sink (OWASP MCP08).

One slog record per call: tool, principal, origin (static/jwt/unknown),
wing/hall when given, outcome, duration. Content is reduced to
content_bytes so the audit line never copies brain content.

Refs #144

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ingestion/internal/mcp/server.go

@@ -16,6 +16,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"net/http"
+	"time"
 
 	"github.com/mathiasbq/hyperguild/ingestion/internal/brainstore"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/capture"
@@ -229,7 +230,9 @@ func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 				ctx = withPrincipal(ctx, principal, viaStatic)
 			}
 		}
+		start := time.Now()
 		out, err := s.handleCall(ctx, p.Name, p.Arguments)
+		logToolCall(ctx, p.Name, p.Arguments, start, err)
 		if err != nil {
 			rpcErr = &rpcError{Code: -32000, Message: err.Error()}
 			break