* feat: upgrade default agent + reviewer model to Opus 4.8
Replace Opus 4.7 with Opus 4.8 (claude-opus-4-8) as the supported
Anthropic model surfaced in the profile editor and used by the main
agent and reviewer graphs. Effort levels (low/medium/high/xhigh/max)
and the high default are unchanged, matching the official Opus 4.8
docs. Updates eval config comment and tests accordingly.
* fix: provider-aware fallback for stale stored model ids
Dropping claude-opus-4-7 from the supported set meant persisted
profile/team-settings still holding it failed the SUPPORTED_MODEL_IDS
check and fell through to default_model_pair() — a cross-provider jump
to the OpenAI global default.
Add provider_fallback_pair: when a stored id is no longer supported but
its provider still has a supported model, resolve to that provider's
newest supported model (anthropic:claude-opus-4-7 -> 4.8), preserving
effort when valid. Resolution order is now: valid stored pair ->
same-provider fallback -> global default_model_pair(). Profile overrides
keep deferring to the team default when no model is set or the provider
is unknown.
