chore: remove the global effort selector in favor of per-model overrides

[luoxuanzao]

Summary

• Removed the global "Effort" dropdown from the input toolbar: the per-model editor in the model selector already offers the same reasoning effort levels, so the global control was redundant. Deleted the EffortSelector class, the effortLevel settings field and default, its toolbar/tab wiring, and thinking-selector.css.
• Runtime paths (cold start, query options builder, dynamic updates, inline edit/refine) now resolve the reasoning effort via resolveModelReasoningEffort: the per-model editor override when set, otherwise the model's server default tier.
• The settings reconciler drops the legacy effortLevel value on load (no migration, per product decision); CHANGELOG and README updated accordingly.

Verification

• npm run typecheck
• npm run lint
• npm run test (3323 passed)
• npm run build
• npm run release:check
• npm run audit:prod
• Tested affected qodercli behavior against a real CLI when applicable (verified in the test vault: toolbar no longer shows the Effort dropdown; the per-model editor still lists effort tiers and applies them)

Safety

• No credentials, private vault content, internal URLs, or personal paths are included
• User-visible changes are documented in CHANGELOG.md