[Feature] Add GITHUB_MODEL env var, resolved model display, and rate-limit surfacing (#822)

[LoackyBit]

Description

When using OpenClaude with CLAUDE_CODE_USE_GITHUB=1, users previously had no way to specify which model to use (unlike every other provider), couldn't see the resolved model ID at startup, and had no visibility into rate-limit quota consumption. This PR addresses all three issues.

Related Issues

Gitlawb#822

Changes

1. GITHUB_MODEL env var support

• Added GITHUB_MODEL env var with higher priority than OPENAI_MODEL (backward-compatible fallback), consistent with GEMINI_MODEL, MISTRAL_MODEL, etc.
• resolveProviderRequest() now reads GITHUB_MODEL → OPENAI_MODEL → default when isGithubMode
• createOpenAIShimClient() propagates GITHUB_MODEL → OPENAI_MODEL if the latter is unset

2. Default model → auto for Copilot API (10% discount)

• normalizeGithubCopilotModel() now maps copilot, empty, and auto to the literal "auto" model ID, which gets the ~10% discount on the Copilot API
• normalizeGithubModelsApiModel() maps auto → gpt-4o (since the Models API doesn't support native "auto")

3. Startup screen shows resolved model

• detectProvider() in StartupScreen.ts now uses resolveProviderRequest() to show the actual model ID (e.g., auto, gpt-4.1) instead of the raw alias github:copilot
• getGithubProviderModel() in ProviderManager.tsx now checks GITHUB_MODEL first

4. Rate-limit header surfacing

• New githubRateLimit.ts singleton that parses x-ratelimit-* headers from GitHub API responses
• Wired into openaiShim.ts to capture headers after every successful response in GitHub mode
• formatTotalCost() in cost-tracker.ts appends the rate-limit summary when available

5. Documentation

• .env.example updated with GITHUB_MODEL and GITHUB_BASE_URL in both system-wide and provider sections

Tests

• 25 tests pass in providerConfig.github.test.ts (added 13 new tests for GITHUB_MODEL, normalizeGithubCopilotModel, and auto mapping)
• 7 tests pass in githubRateLimit.test.ts (new file: parsing, formatting, partial data, reset)
• Build passes ✅

Checklist

• Code follows style guidelines
• Tests added/updated
• Documentation updated
• No new warnings