fix: wire up ACP agent cleanup on connection end to prevent memory leaks

[hendem]

Summary

This PR wires up cleanup to be called when ACP connections end, ensuring memory is properly freed.

Problem

When an ACP connection ends (e.g., IDE closes, client disconnects):

• Event subscription loops (for await in setupEventSubscriptions) run forever
• Session state in ACPSessionManager.sessions Map accumulates indefinitely
• sessionAbortControllers never get cleaned up

This leads to the memory leak crashes reported in #4315 and #5363.

Solution

1. ACP.Agent.dispose() - New method that:

   • Aborts all active event subscriptions via AbortController
   • Clears all session state from the session manager
   • Logs cleanup for debugging

2. AbortController pattern in setupEventSubscriptions() - Each session's event subscription loop now:

   • Has an associated AbortController
   • Checks controller.signal.aborted in the loop
   • Cleans up controller reference in finally block
   • Handles errors gracefully during abort

3. Cleanup wired to connection lifecycle - In acp.ts command:

   • Agent instance is captured when created
   • agent.dispose() is called after stdin ends
   • Ensures cleanup happens even on unexpected disconnection

Changes

File	Change
src/acp/agent.ts	Add dispose(), cleanupSession(), AbortController pattern
src/acp/session.ts	Add remove(), size(), sessionIds(), clear() methods
src/cli/cmd/acp.ts	Capture agent instance and call dispose() on connection end
test/acp/session.test.ts	Tests for session manager cleanup methods

Testing

• 5 tests added for ACPSessionManager
• All tests pass
• Typecheck passes
• Build passes

Related Issues

• #4315 - Memory stays unbounded: ACP session map + compacted tool outputs
• #5363 - opencode eating 70gb of memory?

Related PRs

• #6682 - FileTime session cleanup (by @elithrar)
• #7032 - Bus subscription cleanup (dispose methods for Share, ShareNext, Format, Plugin)