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/manager.ts

@@ -667,7 +667,7 @@ export class MultiProviderAgentManager implements AgentManager {
    * Does not use the conversations table — the message is injected directly
    * as the next resume prompt for the agent's Claude Code session.
    */
-  async sendUserMessage(agentId: string, message: string): Promise<void> {
+  async sendUserMessage(agentId: string, message: string, cwd?: string): Promise<void> {
     const agent = await this.repository.findById(agentId);
     if (!agent) throw new Error(`Agent not found: ${agentId}`);
 
@@ -682,7 +682,7 @@ export class MultiProviderAgentManager implements AgentManager {
     const provider = getProvider(agent.provider);
     if (!provider) throw new Error(`Unknown provider: ${agent.provider}`);
 
-    const agentCwd = this.processManager.getAgentWorkdir(agent.worktreeId);
+    const agentCwd = cwd ?? this.processManager.getAgentWorkdir(agent.worktreeId);
 
     // Clear previous signal.json
     const signalPath = join(agentCwd, '.cw/output/signal.json');