## What broke
Meta-only messages (e.g., `/model`, `/tools`) cause the agent to stuck
forever. After issuing a meta-only command, the agent becomes unresponsive
to subsequent requests like `/new` or regular messages.

The agent logs show the meta-only message is processed, but then the loop
waits indefinitely for a response that never arrives.

## Root cause
In `chat_with_model()` at run_agent.py:3115-3119:
- When `meta_only=True`, `_process_user_message()` is called
- It returns `meta_result` (e.g., "Model changed")
- But the code continues to Line 3136-3148 response processing
- Meta-only messages don't produce LLM responses, so response is None
- The loop waits for `_get_response_content(response)` indefinitely

The original code:
```python
if meta_only:
    meta_result = await self._process_user_message(...)
# Then continues to response loop without returning
```

## Why this fix is minimal
Added 5 lines: immediate return for meta-only path.

```python
if meta_only:
    meta_result = await self._process_user_message(...)
    # Meta-only messages don't produce LLM responses.
    # Return the meta_result directly.
    return meta_result if meta_result else "Processed meta-only message."
```

No changes to regular message handling (meta_only=False path unchanged).
No changes to `_process_user_message()` or `_run_meta_only_handler()`.
No opportunistic refactoring.

## What I tested
Added test suite tests/test_meta_only_stuck_fix.py:
- test_meta_only_returns_immediately
- test_meta_only_does_not_enter_response_loop
- test_meta_only_with_none_response
- test_meta_only_flag_detection
- test_process_user_message_meta_only_calls_handler
- test_chat_with_model_meta_only_exits_early

All tests verify meta-only path returns immediately without stuck.

## What I intentionally did not change
- No changes to regular message handling
- No changes to `_run_meta_only_handler()` implementation
- No changes to response content processing
- No opportunistic refactoring

## Evidence
Before: `/model` → agent stuck, no response, `/new` ignored
After: `/model` → "Model changed" response, agent responsive

Fixes NousResearch#11167