fix: omit transient enabled state from saved presets

[leblancfg]

Problem

Saved presets currently include enabled, which is transient next-turn state. That means /fusion preset save can accidentally persist whether fusion happened to be armed at the moment the preset was saved.

A preset should describe the fusion configuration: worker count, models, thinking levels, budgets, and planner tool mode. It should not silently turn the next request on or off because of stale UI/session state.

Change

• Stop writing enabled from snapshotFusionSettings.
• Keep older presets compatible: if an existing preset already contains enabled, applyFusionPresetSettings still honors it through the normal settings resolver.
• Add a regression assertion that newly saved presets omit enabled and apply without arming fusion.

Verification

• pnpm test -- tests/presets.test.ts
• pnpm run typecheck
• pnpm run format:check
• git diff --check