feat(skillkit,doctor): GitHub Copilot CLI target + doctor probes

[elefthei]

Summary

Adds GitHub Copilot CLI as a first-class skill-install target plus af doctor probes, so af skill install covers Copilot the same way it covers Claude Code.

Track B of the two-PR Copilot integration (Track A is #492, the example agent — independent of this PR).

What this PR lands:

• control-plane/internal/skillkit/target_copilot.go — new copilotTarget that symlinks <canonical current>/ into ~/.copilot/skills/<skill>/. Auto-registers via init(), mirrors target_claude_code.go exactly. Detects the target when ~/.copilot/ exists or the copilot binary is on PATH. Warns on stderr if the same skill is also symlinked into ~/.claude/skills/ (Copilot reads both and dedupes by name).
• control-plane/internal/cli/doctor.go — new CopilotStatus block in the doctor report. Reports: copilot binary + version, ~/.copilot config-dir presence, last logged-in user, detected auth env-source names (COPILOT_GITHUB_TOKEN, GH_TOKEN, GITHUB_TOKEN — names only, never values), and whether gh auth status exits 0. Ships with an explicit disclaimer that "sources present" ≠ "authenticated" — Copilot rejects classic ghp_* PATs and requires a fine-grained PAT with the "Copilot Requests" permission.
• README.md — lists "GitHub Copilot CLI" in the Quick Start skill-install target list.
• research/docs/2026-04-20-*.md — four research docs recording the integration and SDK findings (including the PyPI/repo drift where v0.2.2 doesn't expose AssistantMessageData etc.).

No behaviour change for existing targets; no SDK/server wire changes; no dependencies added.

Testing

• ./scripts/test-all.sh — skipped (no Go/Python SDK surface changed). Ran the scoped tests instead:
• Additional verification:
  • go test ./control-plane/internal/skillkit/... — pass (includes new TestCopilotTarget covering detect / install / uninstall / status / symlink redirection).
  • go test ./control-plane/internal/cli/ -run 'Doctor|Helper' — pass.
  • af doctor --json | jq .copilot smoke-tested on dev machine; reports binary + config-dir state correctly.

Checklist

• I updated documentation where applicable. (README Quick Start + four research/docs/2026-04-20-*.md docs.)
• I added or updated tests. (control-plane/internal/skillkit/target_copilot_test.go + copilot added to the detected/registered lists in skillkit_test.go.)
• I updated CHANGELOG.md — new [Unreleased] entry under ### Added.

Screenshots (if UI-related)

Not UI-related.

Related issues

Pairs with #492 (Copilot example agent, independent — merge in either order).

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