feat: release channels (alpha/beta/stable) via PostHog feature flags

- Add `release_channel` to Settings (Rust + TypeScript)
- Add channel selector in Settings panel (alpha/beta/stable)
- Pass `release_channel` as PostHog person property on identify
- Add `isFeatureEnabled()` helper: alpha always true, beta/stable
  use PostHog flags with hardcoded fallback defaults
- Update `useFeatureFlag` to delegate to PostHog-backed evaluation
  (localStorage overrides still work for dev/QA)
- Create ADR-0042 (supersedes ADR-0017)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/hooks/useTelemetry.test.ts

@@ -13,13 +13,15 @@ vi.mock('../lib/telemetry', () => ({
   teardownSentry: () => mockTeardownSentry(),
   initPostHog: (...args: unknown[]) => mockInitPostHog(...args),
   teardownPostHog: () => mockTeardownPostHog(),
+  updatePostHogIdentify: vi.fn(),
+  setReleaseChannel: vi.fn(),
 }))
 
 const baseSettings: Settings = {
   openai_key: null, google_key: null,
   github_token: null, github_username: null, auto_pull_interval_minutes: null,
   telemetry_consent: null, crash_reporting_enabled: null,
-  analytics_enabled: null, anonymous_id: null, update_channel: null,
+  analytics_enabled: null, anonymous_id: null, update_channel: null, release_channel: null,
 }
 
 describe('useTelemetry', () => {
@@ -50,7 +52,7 @@ describe('useTelemetry', () => {
     renderHook(() =>
       useTelemetry({ ...baseSettings, analytics_enabled: true, anonymous_id: 'test-uuid' }, true)
     )
-    expect(mockInitPostHog).toHaveBeenCalledWith('test-uuid')
+    expect(mockInitPostHog).toHaveBeenCalledWith('test-uuid', 'stable')
   })
 
   it('tears down Sentry when crash reporting is disabled after being enabled', () => {