Add skill-format-migrator agent + Copilot authoring reference

[starlightretailceo]

Summary

• Agent (skill-format-migrator.agent.md): Converts Claude Agent Skills and OpenAI Codex skill definitions into Copilot .agent.md / SKILL.md format. Includes decision flow, platform idiom translation, and validation checklist.
• Skill (skills/copilot-skill-authoring/SKILL.md): Authoring reference — frontmatter contracts for both formats, trigger-description patterns, 17-row concept mapping table (Claude/Codex → Copilot), and file organization conventions.

Motivation

The repo has many "build MCP server in language X" generators but nothing for porting existing agent/skill assets into Copilot format. This meta-contribution lowers the barrier for the community to migrate work from other ecosystems (Claude, Codex, Cursor) into awesome-copilot.

Test plan

• Feed a real Claude SKILL.md through the agent and verify output matches repo conventions
• Feed a Codex skill and verify .agent.md output has valid frontmatter
• Cross-reference concept mapping table against CONTRIBUTING.md

🤖 Generated with Claude Code

[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 2072 --base staged

[github-actions]

✅ Base branch is now set correctly.

Removing the prior block because this PR no longer targets main.

[github-actions]

✅ External plugin PR checks passed

• Changed entries detected: 0
• Workflow state label: ready-for-review

Per-plugin quality summary

Plugin	skill-validator	install smoke test	overall	source tree
none	not_run	not_run	not_run	n/a

No changed external plugin entries were detected in this PR.

[github-actions]

🔒 PR Risk Scan Results

Scanned 4 changed file(s).

Severity	Count
🔴 High	0
🟠 Medium	1
ℹ️ Info	0

Severity	Rule	File	Line	Match
🟠	package-exec-command	docs/README.skills.md	31	| [acreadiness-assess](../skills/acreadiness-assess/SKILL.md)<br />`gh skills install github/awesome-copilot acreadiness-assess` | Run the AgentRC readiness assessment on the curre

This is an automated soft-gate report. Findings indicate review targets and do not block merge by themselves.

[github-actions]

🔍 Skill Validator Results

✅ All checks passed

Scope	Checked
Skills	1
Agents	1
Total	2

Severity	Count
❌ Errors	0
⚠️ Warnings	0
ℹ️ Advisories	0

Summary

Level	Finding
ℹ️	Found 1 skill(s)
ℹ️	[copilot-skill-authoring] 📊 copilot-skill-authoring: 1,991 BPE tokens [chars/4: 2,384] (detailed ✓), 18 sections, 5 code blocks
ℹ️	Found 1 agent(s)
ℹ️	Validated 1 agent(s)
ℹ️	✅ All checks passed (1 skill(s), 1 agent(s))

Full validator output

Found 1 skill(s)
[copilot-skill-authoring] 📊 copilot-skill-authoring: 1,991 BPE tokens [chars/4: 2,384] (detailed ✓), 18 sections, 5 code blocks
Found 1 agent(s)
Validated 1 agent(s)
✅ All checks passed (1 skill(s), 1 agent(s))

[aaronpowell]

The items in this PR are incorrect in many instances. Copilot will load files from the folder structures used by Claude where they are applicable, and does mapping when front matter has inconsistencies. There is no need to create bespoke implementations.