feat: add xAI as official provider (#865)

* feat: add xAI as official provider

- Add xAI preset to ProviderManager (alphabetical order)
- Add xAI provider detection via XAI_API_KEY
- Add xAI startup screen heuristic (x.ai base URL or grok model)
- Add xAI status display properties
- Add grok-4 and grok-3 context windows
- Add xAI model fallbacks across all tiers
- Fix JSDoc priority order in providerAutoDetect

Co-Authored-By: Claude Opus 4.6 <noreply@openclaude.dev>

* fix(xai): persist relaunch classification for xAI profiles

Addresses reviewer feedback on feat/xai-official-provider:
- isProcessEnvAlignedWithProfile now validates XAI_API_KEY for x.ai
  base URLs, mirroring the Bankr pattern. Without this, relaunch
  skips re-applying the profile, XAI_API_KEY stays unset, and
  getAPIProvider() falls back to 'openai'.
- buildOpenAICompatibleStartupEnv now sets XAI_API_KEY when syncing
  active xAI profile to the legacy fallback file.
- Adds 'xai' to VALID_PROVIDERS and --provider xai CLI flag support.
- Adds xAI detection to providerDiscovery label heuristics.
- Adds 'xai' to legacy ProviderProfile type/isProviderProfile guard.
- Adds targeted tests for relaunch alignment, flag application, and
  discovery labeling.

Co-Authored-By: OpenClaude <openclaude@gitlawb.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@openclaude.dev>
Co-authored-by: OpenClaude <openclaude@gitlawb.com>

src/components/StartupScreen.ts

@@ -138,6 +138,7 @@ export function detectProvider(modelOverride?: string): { name: string; model: s
     else if (/api\.kimi\.com/i.test(baseUrl)) name = 'Moonshot AI - Kimi Code'
     else if (/moonshot/i.test(baseUrl)) name = 'Moonshot AI - API'
     else if (/deepseek/i.test(baseUrl)) name = 'DeepSeek'
+    else if (/x\.ai/i.test(baseUrl)) name = 'xAI'
     else if (isZaiBaseUrl(baseUrl)) name = 'Z.AI - GLM'
     else if (/mistral/i.test(baseUrl)) name = 'Mistral'
     // rawModel fallback — fires only when base URL is generic/custom.
@@ -148,6 +149,7 @@ export function detectProvider(modelOverride?: string): { name: string; model: s
     else if (/\bkimi-k/i.test(rawModel) || /moonshot/i.test(rawModel))
       name = 'Moonshot AI - API'
     else if (/deepseek/i.test(rawModel)) name = 'DeepSeek'
+    else if (/grok/i.test(rawModel)) name = 'xAI'
     else if (containsExactZaiGlmModelId(rawModel)) name = 'Z.AI - GLM'
     else if (/mistral/i.test(rawModel)) name = 'Mistral'
     else if (/llama/i.test(rawModel)) name = 'Meta Llama'