feat: add Anthropic (Claude Code) quota provider

[diogoleitao]

Refs #29

Summary

• Adds src/lib/anthropic.ts: credential loading, token refresh via claude CLI, and quota fetch from GET https://api.anthropic.com/api/oauth/usage
• Adds src/providers/anthropic.ts: normalizes 5-hour and 7-day windows into QuotaToastEntry for both classic and grouped toast styles
• Updates src/providers/registry.ts: registers anthropicProvider first in the list
• Updates tests/quota-provider-boundary.test.ts: adds anthropic to the registry snapshot
• Adds tests/lib.anthropic.test.ts: 23 tests covering credential resolution, isNearExpiry, parseUsageResponse, and queryAnthropicQuota (fetch success, 401/403, network error, bad JSON, unexpected shape)
• Adds tests/providers.anthropic.test.ts: 9 tests covering classic/grouped toast entries, worst-window selection, error mapping, matchesCurrentModel, and isAvailable
• Updates README.md: adds Anthropic to the provider list, setup table, and provider-specific notes with attribution

Behaviour

• Provider id: anthropic
• Auto-detected when OpenCode is configured with the anthropic provider
• Credential resolution order: ~/.claude/.credentials.json → macOS Keychain (Claude Code-credentials) → CLAUDE_CODE_OAUTH_TOKEN env var
• Token near expiry (within 5 min): invokes claude CLI for silent refresh — same pattern as opencode-claude-auth
• Classic style: shows the worse of the two windows
• Grouped style: shows both Claude 5h and Claude 7d entries
• Graceful errors on missing credentials, 401/403, network failure

Checks

• npm run typecheck ✓
• npm test ✓ (272 tests pass)
• npm run build ✓
• Tested with OpenCode 1.3.2

Attribution

Credential loading and API endpoint approach based on prior art from claude-lens (Astro-Han) and ClaudeBar (tddworks).

[slkiser]

@diogoleitao I made some changes, could you test locally, since I don't have an anthropic subscription?

I took out the MacOS keychain only because I remember seeing some future feature like this being added to Opencode on a dev branch.

Thanks!