feat: cw task add CLI command + {AGENT_ID} prompt placeholder

- Add `createTaskForAgent` tRPC mutation: resolves agent → task → phase, creates sibling task
- Add `cw task add <name> --agent-id <id>` CLI command
- Replace `{AGENT_ID}` and `{AGENT_NAME}` placeholders in writeInputFiles() before flushing
- Update docs/agent.md and docs/cli-config.md

apps/server/agent/file-io.ts

@@ -282,6 +282,17 @@ export async function writeInputFiles(options: WriteInputFilesOptions): Promise<
     });
   }
 
+  // Replace agent placeholders in all content before writing
+  const placeholders: Record<string, string> = {
+    '{AGENT_ID}': options.agentId ?? '',
+    '{AGENT_NAME}': options.agentName ?? '',
+  };
+  for (const w of writes) {
+    for (const [token, value] of Object.entries(placeholders)) {
+      w.content = w.content.replaceAll(token, value);
+    }
+  }
+
   // Flush all file writes in parallel — yields the event loop between I/O ops
   await Promise.all(writes.map(w => writeFile(w.path, w.content, 'utf-8')));