Add a provider-level disableExperimentalBetas setting that writes CLAUDE_CODE_DISABLE_EXPERIMENTAL_BETAS=1 into managed provider runtime env, keeps stale parent env isolated, and exposes the toggle in desktop provider settings.
Tested: bun test src/server/__tests__/provider-runtime-env.test.ts --test-name-pattern "experimental betas|stale proxy"
Tested: bun test src/server/__tests__/providers.test.ts --test-name-pattern "experimental betas|POST /api/providers should create a provider|PUT /api/providers/:id should update a provider"
Tested: bun test src/server/__tests__/conversation-service.test.ts --test-name-pattern "experimental beta kill switch"
Tested: cd desktop && bun run test -- --run src/__tests__/generalSettings.test.tsx -t "experimental beta headers|Tool Search"
Tested: cd desktop && bun run test -- --run src/lib/__tests__/providerSettingsJson.test.ts
Tested: bun run check:server
Tested: cd desktop && bun run build
Not-tested: bun run verify
Confidence: high
Scope-risk: moderate
2c5c904c0a