Phase 1: split source vs published refs and verify dual publish

[aaronpowell]

Pull Request Checklist

• I have read and followed the CONTRIBUTING.md guidelines.
• I have read and followed the Guidance for submissions involving paid services.
• My contribution adds a new instruction, prompt, agent, skill, workflow, or canvas extension file in the correct directory.
• The file follows the required naming convention.
• The content is clearly structured and follows the example format.
• I have tested my instructions, prompt, agent, skill, workflow, or canvas extension with GitHub Copilot.
• I have run npm start and verified that README.md is up to date.
• I am targeting the staged branch for this pull request.

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Description

Phase 1 needs to preserve contributor flow while making published artifacts resolvable from marketplace. This PR finishes the repo-side wiring so source-content and published-artifact refs are explicit, and dual-publish output stays verifiable.

• Adds explicit source vs published branch constants for raw URL/ref generation.
• Updates README generation to use source-intent install links and documents the tool-facing published manifest on marketplace.
• Adds a post-publish parity check in publish.yml to fail if main and marketplace diverge during the compatibility window.
• Adds a contributor note linking Phase 2 execution to Issue Track migration to marketplace published branch and main source branch #1368 rollout gating.

Fixes: #1368

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Type of Contribution

• New instruction file.
• New prompt file.
• New agent file.
• New plugin.
• New skill file.
• New agentic workflow.
• New canvas extension.
• Update to existing instruction, prompt, agent, plugin, skill, workflow, or canvas extension.
• Other (please specify):

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Additional Notes

Validation run in this branch covered npm run build and npm run plugin:validate. npm run skill:validate still reports a pre-existing repository issue in skills/aws-cloudwatch-investigation (invalid skill name format), unrelated to this change set.

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By submitting this pull request, I confirm that my contribution abides by the Code of Conduct and will be licensed under the MIT License.

[Copilot]

Pull request overview

This PR updates the repository’s “source vs published” branch wiring to support Phase 1 of the migration to a dedicated marketplace published branch while preserving the existing contributor flow.

Changes:

• Introduces explicit constants for source-content vs published-artifact refs and base URLs used for raw-link generation.
• Updates README/plugin docs to surface “source” vs “published” guidance, including a tool-facing published manifest URL on marketplace.
• Adds a post-publish parity check in publish.yml to fail the workflow if main and marketplace published outputs diverge.

Show a summary per file

File	Description
eng/constants.mjs	Adds source/published branch constants and corresponding raw base URLs; updates exports.
eng/update-readme.mjs	Switches install badge URL generation to use source/published base URLs; adds published manifest + source guidance to the plugins section output.
docs/README.plugins.md	Documents the published manifest URL on marketplace and a link to source plugin content.
CONTRIBUTING.md	Adds a note pointing Phase 2 gating to Issue #1368 rollout status.
.github/workflows/publish.yml	Adds a post-push parity check between main and marketplace branches.

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 2