Add Skillpack Forge instructions

[guorunjie]

Summary

Adds a new custom instruction for using Skillpack Forge to create, compile, verify, and package portable AI coding-agent context from one skillpack.yaml.

Why this fits

Skillpack Forge helps Copilot users keep .github/copilot-instructions.md, AGENTS.md, CLAUDE.md, Cursor rules, Claude/Codex Skills, local MCP servers, and MCPB bundles synchronized from one repo manifest.

The instruction focuses on safe, repeatable workflows rather than tool promotion:

• inspect existing files first
• import or initialize the manifest correctly
• dry-run before writing generated files
• run strict checks before claiming files are fresh
• package MCP context only when appropriate

Validation

• Confirmed the repository contribution guidelines for adding instructions.
• Confirmed no existing skillpack-forge.instructions.md entry.
• Added one source instruction file under instructions/.

Note: I used the GitHub API to create this PR because a local shallow clone repeatedly stalled before checkout. If maintainers prefer generated README tables in the same PR, I can add the build output in a follow-up commit.

[github-actions]

⚠️ This PR targets main, but PRs should target staged.

The main branch is auto-published from staged and should not receive direct PRs.
Please close this PR and re-open it against the staged branch.

You can change the base branch using the Edit button at the top of this PR,
or run: gh pr edit 1904 --base staged