…picker

opencode ≥ 1.15.13 advertises its catalog via a `category:"model"` config
option whose entries carry no per-model effort; effort is a sibling
`category:"thought_level"` option opencode only surfaces for the *active*
model. So `translateBackendState` could only attach effortOptions to the
current model, leaving every other picker row with no effort stepper until
it was selected.

Eagerly, right after a session's catalog loads, probe each enabled model
once to learn its effort and cache it for the picker. The probe loop reuses
the existing warm probe session (a model switch is ~ms; creating a session
is ~1s) and runs before the warm entry is published, restoring the original
model in a finally — so the session the manager adopts is never left on a
probed model (race-free). opencode-only via a new optional descriptor hook
`prefetchEffortCatalog`; other backends are untouched.

- descriptor.ts: add optional `prefetchEffortCatalog` to BackendDescriptor
- opencode/descriptor.ts: implement it (skip missing-key models, swallow
  per-model errors, restore original; frozen EMPTY_EFFORT_CATALOG)
- AgentModelPreloader: effortCatalog cache + getEffortCatalog + run prefetch
  in runProbe before publishing warm; clear on clearCached/shutdown
- AgentSessionManager: getEffortCatalog passthrough
- agentModelPickerHelpers: buildEffortOptionsByModelKey falls back to the
  prefetch cache for non-active rows (live effort wins for the active model)
- useAgentModelPicker: include the effort-catalog in the picker re-render signal

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

…ssion

Enabling a new model restarts the backend, but when the active chat tab was
on that backend, restartBackendNow recreated the session without re-probing.
clearCached had already wiped the per-model effort catalog, and a live
session's attachModelCacheSync mirrors catalog state but not the effort
catalog — so effort steppers vanished for every model until a plugin reload.

Couple restart with probing: for an installed backend, always run the
preloader probe (which re-runs the effort prefetch). When a tab on the
backend is active, await the probe and let the replacement session adopt the
warm proc — still a single spawn, with the per-model switch flicker kept on
the throwaway probe session. The only restart-without-probe case left is an
uninstalled backend, which runProbe already self-guards.

Also reformats one unrelated builtinSkills test line via `npm run format`.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>