#13804: fix: prevent steer during tool execution to avoid tool_use/tool_result mismatch (#13704)

## Problem When a long-running exec command is in progress and a queued subagent announcement is delivered, `queueEmbeddedPiMessage` injects the message via `steer` into the active session. This inserts a user message between `tool_use` and `tool_result`, breaking Anthropic's strict pairing requirement and causing API rejection: ``` invalid_request_error: messages.59: `tool_use` ids were found without `tool_result` blocks immediately after: call_52523385. ``` ## Root Cause `queueEmbeddedPiMessage` only checks `isStreaming()` and `isCompacting()` before injecting messages, but doesn't check whether tools are currently executing. When a tool (e.g., exec) is running, the session has an outstanding `tool_use` waiting for its `tool_result`. Injecting a user message at this point breaks the pairing. ## Fix Added `isToolRunning` check to `EmbeddedPiQueueHandle` and `queueEmbeddedPiMessage`. When tools are executing, `queueEmbeddedPiMessage` returns `false`, causing the caller to fall through to the queue path (`enqueueAnnounce`), which delivers the message after the current tool loop completes. The implementation leverages the existing `toolMetaById` map in the subscription state, which already tracks in-flight tools (entries added on `tool_execution_start`, removed on `tool_execution_end`). No new state is needed — just a new accessor `isToolRunning()` that checks `toolMetaById.size > 0`. ### Changes - **`src/agents/pi-embedded-runner/runs.ts`**: Added optional `isToolRunning` to `EmbeddedPiQueueHandle` type; added guard in `queueEmbeddedPiMessage` that returns `false` when tools are running - **`src/agents/pi-embedded-runner/run/attempt.ts`**: Wired `subscription.isToolRunning()` into the `queueHandle` - **`src/agents/pi-embedded-subscribe.ts`**: Exposed `isToolRunning()` from subscription return, backed by `state.toolMetaById.size > 0` Closes #13704