feat: Add idle timeout to kill inactive sessions after 15 minutes

[charlesvien]

1. Add 15-minute idle timeout that auto-kills sessions with no user activity
2. Track activity on permission responses and prompt cycles in AgentService
3. Send heartbeat pings every 5 minutes from TaskLogsPanel when viewing a task
4. Clean up idle-killed sessions in renderer SessionService (unsubscribe, remove state)
5. Clear all timeout handles on service teardown and session cleanup

[charlesvien]

• fix: Reduce agent eagerness to re-enter plan mode mid-task #1208
• feat: Scope oauth tokens by org, replace flat project list with org-keyed map #1210
• feat: Add multi-agent command center grid view #1212
• feat: Add effort level control for Claude sessions #1211
• feat: Optimistically render conversation items before subscription confirms #1209
• feat: Add idle timeout to kill inactive sessions after 15 minutes #1207 👈 (View in Graphite)
• fix: Fix corrupted view redirect, workspace cache flash and diff collapse #1206
• main

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[charlesvien]

Merge activity

• Mar 12, 10:49 PM UTC: A user started a stack merge that includes this pull request via Graphite.
• Mar 12, 10:49 PM UTC: @charlesvien merged this pull request with Graphite.