feat(adapter/gh-copilot): GitHub Copilot CLI and Coding Agent skill runtime

**Goal:** Bring GitHub Copilot CLI (`gh copilot`) and the GitHub Coding Agent to feature parity with Claude Code as a skill runtime. Grounds: [RFC-AI-0004 Principle 3 — Vendor neutrality](../tree/main/docs/rfcs/RFC-AI-0004.md).

**Why GitHub Copilot CLI:** GitHub-native, ubiquitous in OSS workflows, included in many corporate / educational entitlements. The GitHub Coding Agent (autonomous PR-author bot tied to issue assignment) is a different surface from the CLI but is *also* worth covering, because adopter PMCs will encounter both.

**What "parity" means:**

- Skills under `.claude/skills//SKILL.md` are invokable from `gh copilot` (CLI invocation) and consumable by the Coding Agent (as the agent's context / instructions)
- The `tools/*` bridges are reachable — `gh copilot` already has `gh` itself wired in
- Sandbox parity: Coding Agent runs in GitHub-hosted environments; document the mapping to the framework's sandbox baseline (different threat model — sandbox is GitHub's problem, but network/secret handling is still ours)
- HITL primitives: Coding Agent produces draft PRs reviewed by humans — that *is* the HITL boundary; document the alignment with the framework's "draft before send" rule

**Suggested approach:**

- Map `SKILL.md` → `copilot-instructions.md` + `.github/copilot/` (Copilot reads these natively for project-level context)
- For the Coding Agent: skills land as `AGENTS.md` or `.github/copilot-instructions.md`
- Document skill-invocation for the `gh copilot` CLI pathway
- Validate which framework skills make sense for the autonomous Coding Agent vs the interactive CLI — not every skill is appropriate for autonomous execution

**Reference:**

- RFC-AI-0004: [`docs/rfcs/RFC-AI-0004.md`](../tree/main/docs/rfcs/RFC-AI-0004.md)
- `gh copilot` CLI: https://github.com/github/gh-copilot
- Copilot Coding Agent: https://docs.github.com/copilot/concepts/coding-agent
- Copilot instructions / `AGENTS.md`: https://docs.github.com/copilot/customizing-copilot/adding-repository-custom-instructions-for-github-copilot
- Existing skill shape: [`.claude/skills/`](../tree/main/.claude/skills/)