feat: support custom baseURL via opencode.json and ANTHROPIC_BASE_URL…

[anxjok]

Summary

Add support for configuring a custom Anthropic API base URL via opencode.json or the ANTHROPIC_BASE_URL environment variable. This enables routing Anthropic API requests through proxies, API gateways, and custom endpoints while maintaining OAuth authentication through Claude Code credentials.
Key features:

• Configure custom base URL in opencode.json via provider.anthropic.options.baseURL
• Fallback support via ANTHROPIC_BASE_URL environment variable
• Priority chain: config file > environment variable > default Anthropic endpoint
• Custom headers from provider.anthropic.options.headers are automatically forwarded (useful for authentication headers like X-Gateway-Key)
• Fully backward compatible — existing setups continue to work without changes

Related issue

Closes #160 (FR: Anthropic API mode for e.g. CLIProxyAPI)

Testing

• Manually tested with live proxy endpoint:
  • Requests correctly routed to proxy URL (not default API)
  • 200 OK responses received
  • Custom headers (e.g., X-Gateway-Key) forwarded correctly
  • Fallback to default endpoint when no config/env var is set
• Priority chain validated: config > env var > default
• All 225 tests pass (220 existing + 5 new baseURL tests):
  • Lint: 0 warnings, 0 errors
  • Build: TypeScript compilation successful
  • Test suites: 39 suites, 0 failures
• No regression in existing functionality
  Test coverage includes:
• Uses default baseURL when no config or env var is set
• Uses provider.anthropic.options.baseURL from opencode.json when provided
• Uses ANTHROPIC_BASE_URL environment variable when set (no config)
• Prioritizes config over environment variable (correct precedence)
• Does not double /v1 path segment (correct URL merging)

Checklist

• PR title follows Conventional Commits (feat:)
• make all passes locally (runs lint, build, and test)
• Tests added or updated where applicable (5 new baseURL tests in src/index.test.ts)
• README or docs updated where applicable (added "Custom base URL / Proxy support" section and ANTHROPIC_BASE_URL env var documentation)