feat(experiments): add dynamic thinking effort experimental setting (DTE series 1/5)

[easonLiangWorldedtech]

What

Add the dynamicThinkingEffort experimental setting (off by default) that gates the dynamic thinking-effort feature set: the model adjusting its own reasoning effort per turn via a native tool, and the user adjusting it in-chat.

Part of the Dynamic Thinking Effort series (5 PRs). Master design issue: easonLiangWorldedtech#28.

PR protocol (all series PRs): draft PR until CI is fully green -> switch to ready -> CodeRabbit bot review addressed in full -> 100% test coverage on patch lines. Each PR <= 1000 diff lines, based on latest upstream main (stacked on preceding series PRs where noted; rebased to pure main once dependencies merge).

Stacking: PR 1/5 — standalone, based on main.

Scope

• packages/types/src/experiment.ts — experiment id dynamicThinkingEffort
• src/shared/experiments.ts — EXPERIMENT_IDS entry + default config { enabled: false }
• webview-ui/src/components/settings/ExperimentalSettings.tsx — toggle following the existing experimental-setting render pattern
• i18n keys for the new setting in every locale the check-translations CI gate requires

Out of scope

Native tool, task state, and chat UI surfaces — follow-up PRs 2-5 of the series.

Acceptance criteria

• Setting defaults to off; with it off there is zero behavior change (no tool, no UI, no request change)
• Experimental settings view renders the toggle with name/description; check-translations green
• Tests: default config, both on/off cases, UI binding (repo test conventions)
• <= 1000 diff lines; 100% test coverage on patch lines