Trigger main-to-staging after release-notes merges

[cachafla]

What and why?

Release-note previews no longer use paired documentation PRs with temporary ref: overrides. That removed the documentation main push that previously started the normal main → staging → staging deploy chain after a release-notes merge.

Accept a release-notes-published repository dispatch in Merge main into staging. Its successful completion triggers the existing staging deployment through workflow_run, preserving the normal documentation promotion path without creating a temporary ref PR.

The staging deployment run name includes the triggering merge-workflow run ID so the release-notes sender can wait for the exact downstream deployment.

How to test

• Merge this PR before the linked release-notes sender PR.
• Merge a release-notes PR that changes releases/**.
• Confirm Merge main into staging runs with event repository_dispatch.
• Confirm its successful completion triggers Deploy docs site to staging.
• Confirm staging contains the merged release notes and the production artifact is generated.

What needs special review?

• The established main-to-staging workflow remains the staging publication gate.
• Existing push, workflow-run, and manual triggers remain available.
• Production continues to require an artifact from a successful staging workflow.

Dependencies, breaking changes, and deployment notes

Merge this PR before validmind/release-notes#110. No user-facing changes.

Release notes

n/a (internal)

[github-actions]

PR Summary

This PR updates several GitHub Actions workflows and documentation to improve the deployment process for documentation and integrate release-notes management. The main changes include:

• In the deploy-docs-staging workflow, a dynamic run-name has been added to better identify runs using either the workflow run ID or commit SHA.
• The merge-main-into-staging workflow now includes a dynamic run-name and supports an additional trigger via the repository_dispatch event with type release-notes-published. This allows external dispatches (from a release-notes repository, for instance) to trigger the merge process.
• Minor formatting adjustments in the workflow file regarding environment variable configuration for clarity.
• Updates to the AGENTS.md documentation clarify that release-note content is sourced from a separate repository (validmind/release-notes). Instead of generating a documentation pull request, a dispatch event triggers the existing merge workflow which is responsible for staging deployment. This clarification helps explain the modified flow for handling release notes and documentation promotion.

Test Suggestions

• Trigger the deploy-docs-staging workflow manually (or simulate a push) to verify that the run-name is properly assigned and reflects the expected dynamic value.
• Dispatch a repository_dispatch event with the type 'release-notes-published' to ensure that the merge-main-into-staging workflow is activated correctly.
• Verify that the merge workflow deletes the temporary branch as expected when the job completes successfully.
• Check the updated documentation in AGENTS.md to ensure that it accurately reflects the new release-notes triggered deployment process.

[github-actions]

Validate docs site

✓ INFO: A live preview of the docs site is available — Open the preview

[github-actions]

Lighthouse check results

✓ INFO: No site pages to audit in this PR.

Commit SHA: a9b37e1