fix(gateway): recover from hung agents — /stop hard-kills session lock

[teknium1]

Summary

Salvage of PR #2498 by @Mibayy onto current main.

When an agent thread hangs (truly blocked, never checks _interrupt_requested), /stop now force-cleans _running_agents to unlock the session immediately. Previously, /stop called agent.interrupt() which sets a flag the hung agent never reads — the session stayed locked forever, showing "writing..." with no output.

Changes

Early /stop intercept — New block in the running-agent guard (following the existing /new intercept pattern) that catches /stop, calls interrupt() on the agent, then force-deletes the entry from _running_agents and clears pending messages. Returns immediately with a confirmation.

Sentinel /stop force-clean — /stop during agent startup now force-cleans the sentinel instead of returning "nothing to stop yet", so the session actually unlocks.

10-minute hard timeout — Wraps loop.run_in_executor() in asyncio.wait_for(timeout=600). On timeout, interrupts the agent and constructs a synthetic response. The thread keeps running (Python can't kill threads) but the session lock is released.

Follow-up improvements over original PR

• Consolidated duplicate resolve_command imports — single early resolution shared by /stop and /new intercepts
• Updated _handle_stop_command() to also force-clean for consistency (both paths now behave identically)
• Added zombie thread documentation on the timeout handler

Tests

• Updated test 6 (sentinel /stop) to verify force-cleanup
• Added test 6b: /stop hard-kills a running agent
• Added test 6c: /stop clears pending messages

All 6178 tests pass.

Closes #2491. Cherry-picked from #2498 by @Mibayy.

[github-actions]

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