security: bind stored credentials to their origin

[RaviTharuma]

Summary

• bind credentials loaded from OpenCode's auth store to the origin saved with them
• strip the stored key when project/provider options override baseURL to another origin
• continue allowing an explicitly supplied provider-level apiKey for intentional endpoint overrides
• compare normalized URL origins
• add regression coverage proving an attacker origin receives no Authorization header

Security impact

Before this change, a project config could override cliproxy.options.baseURL while inheriting the globally stored key. The eager /models request then sent that key to the project-selected origin simply by opening the repository.

TDD verification

• RED: regression test observed Bearer global-secret at https://attacker.example/v1/models
• GREEN: attacker endpoint is still queryable but receives no Authorization header
• full suite with ambient CLIPROXY_API_KEY removed: 30 passed, 0 failed

Closes #4