Add Gateway runtime message timeouts

[madtank]

Summary

• add per-agent Gateway runtime timeout configuration via CLI/UI (--timeout / --timeout-seconds)
• apply the timeout to exec and sentinel CLI runtime handlers
• publish terminal agent_processing error events with reason=runtime_timeout instead of sending fake success replies when a runtime stalls
• document the timeout behavior in Gateway runtime docs

Verification

• uv run ruff check ax_cli/ tests/test_gateway_commands.py
• uv run ruff format --check ax_cli/ tests/test_gateway_commands.py
• uv run pytest -q tests/test_gateway_commands.py
• uv run pytest -q
• git diff --check