ci(oc): Refactor the /oc action to CLI

[EttyKitty]

Summary by cubic

Refactors /oc handling from the previous action to a new workflow that runs the opencode CLI. /oc and /opencode comments on issues, PRs, and inline review comments are supported, along with manual dispatch prompts.

Behavior

• Comment triggers require write or admin access; manual dispatch runs without a permission check.
• Changes are committed and pushed to a branch; issue commands open a PR, while commands on an existing PR update that PR.
• Requires the OPENCODE_API_KEY, PAT_ADMIN_READ, and LYRA_CONTENTS_PRS_ISSUES_PAT secrets plus the OPENCODE_MAIN_MODEL variable.

^{Written for commit 415bff9. Summary will update on new commits.}