[Bug] Effort level change via /effort command affects all active sessions globally

## Summary

/effort level changes in one Claude Code session unexpectedly propagate to all other active sessions when using Opus 4.7. Effort appears to be stored/read globally rather than per-session.

## Environment

• Platform: darwin
• Terminal: iTerm.app
• Version: 2.1.119
• Feedback ID: fce0a0ea-346a-437b-8e7f-b28c6274c359
• Model: Opus 4.7 (claude-opus-4-7)
• Multiple concurrent Claude Code sessions in separate terminal windows

## Steps to Reproduce

1. Open Terminal A, start new Claude Code session with Opus 4.7
2. Run /effort xhigh in Terminal A
3. Begin working on a prompt in Terminal A (long-running task)
4. Open Terminal B with a separate Claude Code session (also Opus 4.7)
5. Observe: Terminal B's ongoing task is now running at xhigh effort
6. In Terminal B, run /effort high
7. Switch back to Terminal A

## Expected Behavior

Each session maintains its own independent effort level. Changing /effort in one session should not affect any other session.

## Actual Behavior

Terminal A's effort level has changed to high, matching the value set in Terminal B. Effort level is shared across all active sessions instead of being session-scoped.

## Impact

• Cannot run concurrent sessions at different effort levels
• A background long-running task can have its effort silently changed mid-run by activity in another window
• No warning or indication that the change is global