fix: use public billing API for Copilot quota after OpenCode partnership change

[Bittabola]

Summary

OpenCode's new OAuth integration (Jan 2026 partnership with GitHub) doesn't grant access to the internal /copilot_internal/* endpoints, causing quota queries to return 404 errors. This PR adds support for querying quota via GitHub's public billing API.

Changes

• Add CopilotQuotaConfig type with token, username, and tier fields
• Add CopilotTier type for subscription tiers: free, pro, pro+, business, enterprise
• Read config from ~/.config/opencode/copilot-quota-token.json
• Call public API: GET /users/{username}/settings/billing/premium_request/usage
• Format response with progress bar, model breakdown, and billing period
• Fall back to internal API for legacy tokens (backward compatible)
• Add helpful setup instructions in error messages (English & Chinese)

Tier Limits

Tier	Monthly Premium Requests
free	50
pro	300
pro+	1500
business	300
enterprise	1000

Setup

Users need to create a fine-grained PAT with "Plan" read permission:

1. Visit https://github.com/settings/tokens?type=beta
2. Create new token with "Account permissions" → "Plan" → "Read-only"
3. Create config file ~/.config/opencode/copilot-quota-token.json:

{
  "token": "github_pat_xxx...",
  "username": "YourUsername",
  "tier": "pro"
}

Example Output

Account:        GitHub Copilot (@username)

Premium        ████████████████░░░░ 80% (60/300)

Model breakdown:
  Claude Opus 4.5: 60 requests

Period: 2026-01