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/useFeatureFlag.ts

@@ -1,19 +1,15 @@
 /**
- * Local feature flag hook (V1 — no remote fetching).
+ * Feature flag hook backed by PostHog + release channels.
  *
  * Flags are resolved in order:
  *   1. localStorage override (`ff_<name>`) — for dev/QA testing
- *   2. Compile-time defaults in FLAG_DEFAULTS
- *
- * To add a new flag: add its name to the FeatureFlagName union and
- * set its default in FLAG_DEFAULTS.
+ *   2. PostHog feature flags (evaluated by release channel)
+ *   3. Alpha channel always returns true (sees all features)
  */
 
-export type FeatureFlagName = 'example_flag'
+import { isFeatureEnabled } from '../lib/telemetry'
 
-const FLAG_DEFAULTS: Record<FeatureFlagName, boolean> = {
-  example_flag: false,
-}
+export type FeatureFlagName = 'example_flag'
 
 export function useFeatureFlag(flag: FeatureFlagName): boolean {
   try {
@@ -22,5 +18,5 @@ export function useFeatureFlag(flag: FeatureFlagName): boolean {
   } catch {
     // localStorage may be unavailable in some contexts
   }
-  return FLAG_DEFAULTS[flag] ?? false
+  return isFeatureEnabled(flag)
 }

src/hooks/useSettings.test.ts

@@ -15,6 +15,7 @@ const defaultSettings: Settings = {
   analytics_enabled: null,
   anonymous_id: null,
   update_channel: null,
+  release_channel: null,
 }
 
 const savedSettings: Settings = {
@@ -28,6 +29,7 @@ const savedSettings: Settings = {
   analytics_enabled: null,
   anonymous_id: null,
   update_channel: null,
+  release_channel: null,
 }
 
 let mockSettingsStore: Settings = { ...defaultSettings }

src/hooks/useSettings.ts

@@ -18,6 +18,7 @@ const EMPTY_SETTINGS: Settings = {
   analytics_enabled: null,
   anonymous_id: null,
   update_channel: null,
+  release_channel: null,
 }
 
 export function useSettings() {

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', () => {

src/hooks/useTelemetry.ts

@@ -1,7 +1,7 @@
 import { useEffect, useRef } from 'react'
 import { invoke } from '@tauri-apps/api/core'
 import { isTauri, mockInvoke } from '../mock-tauri'
-import { initSentry, teardownSentry, initPostHog, teardownPostHog } from '../lib/telemetry'
+import { initSentry, teardownSentry, initPostHog, teardownPostHog, updatePostHogIdentify, setReleaseChannel } from '../lib/telemetry'
 import type { Settings } from '../types'
 
 function tauriCall(command: string): Promise<void> {
@@ -29,13 +29,18 @@ export function useTelemetry(settings: Settings, loaded: boolean): void {
       tauriCall('reinit_telemetry').catch(() => {})
     }
 
+    const channel = settings.release_channel ?? 'stable'
+    setReleaseChannel(channel)
+
     if (analyticsEnabled && id && !prevAnalytics.current) {
-      initPostHog(id)
+      initPostHog(id, channel)
     } else if (!analyticsEnabled && prevAnalytics.current) {
       teardownPostHog()
+    } else if (analyticsEnabled && id) {
+      updatePostHogIdentify(channel)
     }
 
     prevCrash.current = crashEnabled
     prevAnalytics.current = analyticsEnabled
-  }, [loaded, settings.crash_reporting_enabled, settings.analytics_enabled, settings.anonymous_id])
+  }, [loaded, settings.crash_reporting_enabled, settings.analytics_enabled, settings.anonymous_id, settings.release_channel])
 }