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

docs/cli-config.md

@@ -44,6 +44,7 @@ Uses **Commander.js** for command parsing.
 | `list --parent\|--phase\|--initiative <id>` | List tasks with counts |
 | `get <taskId>` | Task details |
 | `status <taskId> <status>` | Update status |
+| `add <name> --agent-id <id> [--description <desc>] [--category <cat>]` | Create sibling task in agent's phase |
 
 ### Dispatch (`cw dispatch`)
 | Command | Description |