fix(acp): close notify channel on EOF to prevent stream hang

[masami-agent]

Summary

Closes #295

When the ACP child process dies (EOF on stdout), the reader task in connection.rs fails to close the notify channel. This causes rx.recv() in the streaming loop to hang forever, holding the per-connection mutex and permanently leaking the pool slot.

Changes

File	Change
src/acp/connection.rs	Fix EOF handler: *sub = None instead of drop(sub) — drops the UnboundedSender, closing the channel
src/adapter.rs	Add 10-minute timeout to rx.recv() as defense-in-depth

Root Cause

In the reader task EOF handler:

// Before (bug)
let sub = notify_tx.lock().await;
drop(sub);  // drops MutexGuard, NOT the Option<Sender> inside

drop(sub) only drops the MutexGuard<Option<UnboundedSender>>. The Option remains Some(sender), so the UnboundedSender is never dropped and rx.recv() never returns None.

// After (fix)
let mut sub = notify_tx.lock().await;
*sub = None;  // drops the Sender → channel closes → rx.recv() returns None

Defense-in-Depth

Added a 10-minute timeout around rx.recv() in the streaming loop (adapter.rs). If no notification arrives within 10 minutes, the loop breaks with an "Agent stopped responding" error. This prevents indefinite hangs even if other channel-closing bugs exist.

Scope Note

Issue #295 also describes a global write lock problem in with_connection. That was already fixed in a prior refactor — with_connection now uses a read lock + per-connection mutex. See validation comment for details. This PR only addresses the remaining confirmed bug (notify channel EOF).

Validation

• Code-level verification: *sub = None drops the UnboundedSender, which is the standard tokio mpsc channel close mechanism
• tokio::time::timeout wrapping rx.recv() is a well-established pattern for stall detection

[github-actions]

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[obrutjack]

Merge checklist verified:

• ✅ CI all green (cargo check + 7 Docker smoke tests)
• ✅ MERGEABLE, upstream branch
• ✅ Fixes real bug: EOF didn't close notify channel → stream_prompt hangs forever
• ✅ 10min timeout as safety net for unresponsive agents
• ✅ Minimal change (+13/-4), correct logic
• ✅ No version regression

Pending @thepagent code owner review.