docs(#2988): add guidance for adding plugins to existing workspaces

[fullsend-ai-coder]

Add a new "Adding a Plugin to an Existing Workspace" subsection to AGENTS.md under "Working with Workspaces". This addresses a gap that caused PR #2977 to fail /publish: the code agent verified plugin existence at the upstream repo's HEAD (via --depth 1 clone) instead of at the pinned repo-ref in source.json.

The new section instructs agents to: 1. Read source.json to get the pinned repo-ref 2. Verify the plugin exists at that ref using the GitHub API 3. Update repo-ref if the plugin is missing at the current ref 4. Check repo-backstage-version consistency when updating refs 5. Add the plugin entry and metadata following existing patterns

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Closes #2988

Post-script verification

• Branch is not main/master (agent/2988-agents-md-add-plugin-guidance)
• Secret scan passed (gitleaks — c2c1dfe6312cd8b965b696945aa8bacdbe3c6ffc..HEAD)
• Pre-commit hooks passed (authoritative run on runner)
• Tests ran inside sandbox

[openshift-ci]

Hi @fullsend-ai-coder[bot]. Thanks for your PR.

I'm waiting for a redhat-developer member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud