feat: send additional_permissions in token exchange request (#859)

* feat: send additional_permissions in token exchange request

Parse the ADDITIONAL_PERMISSIONS env var and send it as a JSON body
in the OIDC token exchange request. Permissions are merged on top of
the standard defaults (contents: write, pull_requests: write,
issues: write).

* docs: list specific available additional permissions

docs/configuration.md

@@ -172,9 +172,14 @@ jobs:
 
 **Important Notes**:
 
-- The GitHub token must have the `actions: read` permission in your workflow
+- The GitHub token must have the corresponding permission in your workflow
 - If the permission is missing, Claude will warn you and suggest adding it
-- Currently, only `actions: read` is supported, but the format allows for future extensions
+- The following additional permissions can be requested beyond the defaults:
+  - `actions: read`
+  - `checks: read`
+  - `discussions: read` or `discussions: write`
+  - `workflows: read` or `workflows: write`
+- Standard permissions (`contents: write`, `pull_requests: write`, `issues: write`) are always included and do not need to be specified
 
 ## Custom Environment Variables