Regression: `/effort` now persists globally; previously was per-session

Bug Description

Setting /effort high (or xhigh) used to apply only to the current session and reset on next session start. Currently it writes effortLevel to settings.json and persists across sessions, forcing a manual reset each time.

Expected: parity with fastModePerSessionOptIn — either restore prior per-session behaviour, or add an effortLevelPerSessionOptIn flag so users can opt back in.

This effectively reduces the workflow to single-threaded whenever you want claude to put more effort into a single item 👎

Repro:

_Regression 1:_
Silently resets effort _globally, with no notice_. Have two sessions A and B open with effort set to medium. In session A, run /effort high. Observe that in session B, effort is immediately set to high instead of remaining medium as expected.

Regression 2:

Persists across sessions, undesirable:

/effort high → exit → relaunch → effort still high.

Environment Info

• Platform: darwin
• Terminal: ghostty
• Version: 2.1.126, 2.1.128
• Feedback ID: 84d16753-3ab3-40cf-8e02-c32db911ed71

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