fix: suppress idle hooks during active subagents

[shanebishop1]

Closes #5.

Summary

• Add opt-in excludeSubagentWait filtering for session.idle hooks.
• Track active task calls per session and suppress only opted-in hooks until every call finishes.
• Preserve existing idle-hook behavior when the option is omitted.
• Clear tracked state when task calls finish or sessions are deleted.
• Document the notification use case and cover concurrent subagents with regression tests.

Configuration

{
  "id": "waiting-for-input",
  "when": {
    "event": "session.idle",
    "excludeSubagentWait": true
  },
  "run": ["notify-user.sh 'Waiting for input'"]
}

Verification

• npm run lint
• npm run typecheck
• npx --yes bun@1.3.9 test (130 passed, 3 gated skips)
• npm run build