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/agent/loop_context.go

@@ -142,6 +142,9 @@ func (l *Loop) injectContext(ctx context.Context, req *RunRequest) (contextSetup
 	if req.TeamID != "" {
 		ctx = tools.WithToolTeamID(ctx, req.TeamID)
 	}
+	if req.LeaderAgentID != "" {
+		ctx = tools.WithLeaderAgentID(ctx, req.LeaderAgentID)
+	}
 
 	// Team workspace: auto-resolve for agents with team membership (not dispatched).
 	// Lead agents default to team workspace; non-lead members keep own workspace.
@@ -256,6 +259,7 @@ func (l *Loop) injectContext(ctx context.Context, req *RunRequest) (contextSetup
 		WorkspaceChannel:    req.WorkspaceChannel,
 		WorkspaceChatID:     req.WorkspaceChatID,
 		TeamTaskID:          req.TeamTaskID,
+		LeaderAgentID:       tools.LeaderAgentIDFromCtx(ctx),
 		AgentToolKey:        l.id,
 	}
 	ctx = store.WithRunContext(ctx, rc)