Restructure Packer into build-machine-image and publish-metadata-to-hcp use cases

[rchandnaWUSTL]

Summary

• Split Packer skills into two use-case folders following the product/use-case/skills/ structure
• build-machine-image/ for AWS, Azure, and Windows image building
• publish-metadata-to-hcp/ for HCP Packer registry integration

Structure

packer/
├── build-machine-image/
│   ├── .claude-plugin/plugin.json
│   └── skills/
│       ├── aws-ami-builder/
│       ├── azure-image-builder/
│       └── windows-builder/
└── publish-metadata-to-hcp/
    ├── .claude-plugin/plugin.json
    └── skills/
        └── push-to-registry/

Changes

• Created build-machine-image/ with 3 builder skills
• Created publish-metadata-to-hcp/ with HCP registry skill
• Each use case has its own plugin.json
• Updated README with new structure and installation instructions

[hashicorp-cla-app]

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2 out of 3 committers have signed the CLA.

• gautambaghel
• rchandnaWUSTL
• Roshan Chandna

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[gautambaghel]

@rchandnaWUSTL - isn't there a better link than this?

something like https://developer.hashicorp.com/terraform/plugin/framework more aligned to the use case?

[gautambaghel]

@rchandnaWUSTL - same as above, shouldn't there be a better link?

[github-actions]

Tessl Skill Review Results

Skill	Status	Review Score
packer/build-machine-image/skills/aws-ami-builder	PASSED	89%
packer/build-machine-image/skills/azure-image-builder	PASSED	81%
packer/build-machine-image/skills/windows-builder	PASSED	94%
packer/publish-metadata-to-hcp/skills/push-to-registry	PASSED	81%

Checks: frontmatter validity, required fields, body structure, examples, line count.
Review Score is informational — not used for pass/fail gating.