feat(agents): per-model reasoning effort level

Add a per-model/key reasoning effort setting to agent definitions,
surfaced and editable in the Agent Settings page and displayed at a
glance in the model selector views.

- core: single source of truth for effort levels (REASONING_EFFORTS,
  DEFAULT_REASONING_EFFORT='high', labels, isReasoningEffort guard);
  add 'xhigh' level; AgentModelEntry.effort; xhigh budget=24000 for
  classic-thinking Claude; default floor 'high'. Persist/parse effort
  in the agent TOML loader.
- api: thread effort through the fallback chain with per-model -> per-tab
  -> default precedence; validate /chat + agentModels effort from the
  canonical list.
- frontend: effort <select> per model row in AgentBuilder; effort badges
  in ModelSelector (agent + subagent chains); Thinking dropdown sourced
  from canonical list; per-tab default raised to 'high'.
- tests: +15 (loader round-trip, agent xhigh budget, canonical list +
  guard, api precedence, route validation).

0 files changed, 0 insertions, 0 deletions