fix: resolve errand worktree path for sendMessage instead of using agent-workdirs

Errand agents are spawned in the errand's git worktree (managed by
SimpleGitWorktreeManager), not in agent-workdirs/<alias>/.
sendUserMessage was deriving the cwd from worktreeId which pointed to
the non-existent agent-workdirs path. Now the errand.sendMessage
procedure resolves the actual worktree path and passes it through.

apps/server/agent/mock-manager.ts

@@ -534,7 +534,7 @@ export class MockAgentManager implements AgentManager {
    * Deliver a user message to a running errand agent.
    * Mock implementation: no-op (simulates message delivery without actual process interaction).
    */
-  async sendUserMessage(agentId: string, _message: string): Promise<void> {
+  async sendUserMessage(agentId: string, _message: string, _cwd?: string): Promise<void> {
     const record = this.agents.get(agentId);
     if (!record) {
       throw new Error(`Agent '${agentId}' not found`);