fix(frontend): honest effort display in agent UI (gemini review)

Address two UI-accuracy issues found in review:
- AgentBuilder: the per-model effort select no longer disguises an unset
  value as 'High'. Adds an explicit 'Inherit' option; choosing it strips
  the effort key so the saved TOML omits it (and the call site falls back
  to per-tab → default), matching displayed intent to persisted state.
- ModelSelector: effort badges for models without an explicit override now
  reflect the actual effective effort (per-tab selector → default) instead
  of always showing the default constant, mirroring backend resolution.

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