feat(team): share leader memory with team members (read-only)

Team members dispatched via team tasks can now read the leader's memory
files (MEMORY.md, memory/*.md) as a fallback when they have no own
memory. Memory writes are blocked for members — only the leader can
save memory files. This follows the existing team_workspace context
propagation pattern: dispatch metadata → RunRequest → context → tools.

Affected tools: read_file (interceptor), memory_get, memory_search,
list_files (interceptor). All include leader fallback with per-user →
global scope cascade.

internal/tools/context_keys.go

@@ -356,6 +356,27 @@ func TeamTaskIDFromCtx(ctx context.Context) string {
 	return ""
 }
 
+// --- Leader agent ID propagation (team task dispatch → memory interceptor) ---
+
+const ctxLeaderAgentID toolContextKey = "tool_leader_agent_id"
+
+// WithLeaderAgentID injects the team leader's agent UUID string into context
+// so the memory interceptor can fallback-read leader's memory for team members.
+func WithLeaderAgentID(ctx context.Context, id string) context.Context {
+	return context.WithValue(ctx, ctxLeaderAgentID, id)
+}
+
+// LeaderAgentIDFromCtx returns the leader's agent UUID string from context.
+func LeaderAgentIDFromCtx(ctx context.Context) string {
+	if v, _ := ctx.Value(ctxLeaderAgentID).(string); v != "" {
+		return v
+	}
+	if rc := store.RunContextFromCtx(ctx); rc != nil {
+		return rc.LeaderAgentID
+	}
+	return ""
+}
+
 // --- Workspace scope propagation (delegation origin) ---
 
 const (