- Adds `_API_BASE_URL` to `CODER_EXTERNAL_AUTH_CONFIG_`
- Extracts and refactors existing GitHub PR sync logic to new packages
`coderd/gitsync` and `coderd/externalauth/gitprovider`
- Associated wiring and tests
Created using Opus 4.6
**Breaking Change:** Existing oauth apps might now use PKCE. If an
unknown IdP type was being used, and it does not support PKCE, it will
break.
To fix, set the PKCE methods on the external auth to `none`
```
export CODER_EXTERNAL_AUTH_1_PKCE_METHODS=none
```
Solves #15575
Adds OAuth access token revocation when unlinking external auth
provider. Due to revocation not being consistently implemented by
providers this is only best effort attempt. Unsuccessful revocation
won't influence link removal.
Cian Johnston