What does this PR do?

Adds an agent_loop_stopped plugin hook fired from gateway/run.py::_interrupt_and_clear_session right after running_agent.interrupt(). Lets plugins drop per-turn external resources (e.g. an outbound RPC call the agent loop was awaiting a result from) when the user interrupts the run with /stop or the fast-path of /new.

Today plugins can react to on_session_finalize for the /new slow path, but /stop and the /new fast-path inside _dispatch_message bypass that hook entirely — outbound work just sits until it times out, while the agent has already moved on.

Related Issue

Fixes #27206

Type of Change

• ✨ New feature (non-breaking change that adds functionality)

Changes Made

• gateway/run.py — emit agent_loop_stopped via invoke_hook right after running_agent.interrupt() in _interrupt_and_clear_session. Wrapped in try/except so a misbehaving handler can't block the interrupt. Same dispatch shape as on_session_finalize at gateway/run.py:8310.
• hermes_cli/plugins.py — add agent_loop_stopped to VALID_HOOKS with inline docstring covering kwargs (session_key, platform, reason, invalidation_reason).
• tests/gateway/test_agent_loop_stopped_hook.py — three pytest cases covering hook presence in VALID_HOOKS, dispatch with expected kwargs on interrupt, and fault tolerance when a registered handler raises.

How to Test

./venv/bin/python -m pytest tests/gateway/test_agent_loop_stopped_hook.py -v

All three cases pass. End-to-end smoke against hermes-livekit (the motivating consumer) is pending v0.3.0 release of that plugin; the plugin's hook subscription is wired but is currently a no-op against an unpatched core.

Motivating use case

hermes-livekit v0.3.0 exposes client-registered remote tools the agent can call over a WebRTC data channel:

# adapter.py — the proxy hermes invokes
async def proxy(args, **_kwargs):
    ...
    return await asyncio.wait_for(future, timeout=TOOL_CALL_TIMEOUT)

When the user hits /stop, the agent's interrupt flag flips but the proxy keeps waiting; the client stays in "owe a result" mode for 30s and the agent:tool-call-cancelled notification never goes out. This hook gives the plugin one signal to fail the future and notify the client immediately.

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (feat(gateway):)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this feature
• I've run pytest tests/gateway/test_agent_loop_stopped_hook.py -v and all 3 tests pass
• I've added tests for my changes
• I've tested on my platform: macOS 25.4.1, Python 3.11

Documentation & Housekeeping

• I've updated relevant documentation — VALID_HOOKS in hermes_cli/plugins.py has an inline docstring describing the new hook's kwargs. No separate hooks documentation file exists; the inline comments next to VALID_HOOKS are the canonical source today.
• I've updated cli-config.yaml.example if I added/changed config keys — N/A (no config keys)
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — N/A (additive plugin hook, no workflow change)
• I've considered cross-platform impact — N/A (pure Python, no platform-specific code)
• I've updated tool descriptions/schemas if I changed tool behavior — N/A (no tools touched)