AI merge review could park forever when the host fusion extension loaded
fn_task_show and booted a second TaskStore without a tool timeout (FN-7956).
- Skip host @runfusion/fusion extensions for sessionPurpose "merger"
- Forward sessionPurpose into createFnAgent for that policy
- Coalesce + 30s-bound extension TaskStore boots; ALS-propagate AbortSignal
- Wrap every extension registerTool execute with 60s timeout/abort fail-closed
- Unit tests for merger host-extension skip and tool timeout helpers
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