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/utils/providerDiscovery.ts

@@ -206,6 +206,10 @@ export function getLocalOpenAICompatibleProviderLabel(baseUrl?: string): string
     if (host.includes('bankr') || haystack.includes('bankr')) {
       return 'Bankr'
     }
+    // xAI Grok endpoint
+    if (host.includes('x.ai') || haystack.includes('x.ai')) {
+      return 'xAI'
+    }
     // Z.AI GLM Coding Plan
     if (isZaiBaseUrl(parsed.href)) {
       return 'Z.AI - GLM'