[FEATURE]: allow runs started by a GitHub App's comment (assertPermissions refuses every App actor)

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

opencode github run refuses every run started by a GitHub App's comment, and there is no way to allow one.

assertPermissions() in packages/opencode/src/cli/cmd/github.handler.ts (v1.18.29, lines 1165-1185) asks repos.getCollaboratorPermissionLevel for github.context.actor on every user event. A GitHub App has no collaborator permission level on a repository, so an App that comments /opencode on an issue always gets:

Asserting permissions for user sreagent-app[bot]...
  permission: none
Error: User sreagent-app[bot] does not have write permissions

This happens in App mode and in token mode (use_github_token: true) alike, and no input or environment variable skips the check. The check is skipped only for schedule and workflow_dispatch.

Our integration (SRE Agent, https://github.com/segfaultpw/sre-agent-opencode) is an App installed on the repository with Issues and Pull requests write. It opens a tracking issue holding a brief and comments /opencode on it, the same way a person would, and every one of those runs is refused. We work around it by relaying the comment through workflow_dispatch, which costs the run its issue context (no reaction, no comment on the issue, a dispatch branch name, PROMPT required).

Request, either of these:

1. Treat an actor of type Bot as authorized when its App has an installation on the repository (GET /repos/{owner}/{repo}/installation, or the installation the token already carries), instead of reading a collaborator level that cannot exist for it.
2. An explicit input on the action, for example allowed_actors: "sreagent-app[bot]", set by the workflow author. The workflow file already decides whose comment reaches the action at all, and the repository's admins own that file.

Happy to open a PR for whichever you prefer.