Add Skillpack Forge instructions

[guorunjie]

Summary

Adds a Skillpack Forge custom instruction for repositories that want one portable agent-context source of truth.

What it helps users do

• Create or import a skillpack.yaml manifest.
• Generate AGENTS.md, CLAUDE.md, Copilot instructions, Cursor rules, Claude/Codex Skills, MCP resources, and MCPB bundles.
• Validate generated context with doctor, diff, and check --strict.
• Keep generated files synchronized in CI with the Skillpack Forge GitHub Action.

Validation

• File follows the instructions/*.instructions.md convention.
• The workflow uses published npx skillpack-forge@latest commands.
• The instructions emphasize inspecting existing files before overwriting anything.

[github-actions]

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[guorunjie]

Reviewer note: Skillpack Forge is now published as skillpack-forge@1.7.0 on npm and has a live fixture repo showing the generated outputs end to end: https://github.com/guorunjie/skillpack-forge-demo. The fixture checks in skillpack.yaml, generated Copilot instructions, AGENTS.md, CLAUDE.md, Claude/Codex skills, Cursor rules, MCP files, an MCPB bundle, and has passing CI.