Post-merge PR comment: templates auto-deploy notice; modules require manual release tagging

Proposal: After a PR is merged, automatically leave a comment on the PR to clarify publish behavior.

Why: Improves contributor feedback by telling them what happens next.

Behavior:
- If changes include registry/**/templates/**: comment that the template will be automatically published to registry.coder.com within ~5–10 minutes (triggered by deploy-registry.yaml on push to main).
- If changes include registry/**/modules/**: comment that a manual release tag is required to publish, and that maintainers will run ./scripts/tag_release.sh (or tag manually) to trigger deployment.

Implementation sketch:
- GitHub Action on pull_request (types: closed) with check github.event.pull_request.merged == true.
- Use the REST API to list files in the PR and detect whether templates and/or modules paths were touched.
- Use actions/github-script (or octokit) to post the appropriate comment(s) with links to MAINTAINER.md and .github/workflows/deploy-registry.yaml.

Acceptance criteria:
- On merge of a templates-only PR, the bot comments with the auto-deploy timing message.
- On merge of a modules-only PR, the bot comments with manual tagging instructions.
- On mixed PRs, the bot posts both notes or a combined message.
- Links included: MAINTAINER.md, .github/workflows/deploy-registry.yaml.