Document GitHub Copilot install requirements #485

Problem

GitHub Copilot has no adept marketplace path, so a user must hand-place adept's files, and nothing
in the repository records what GitHub Copilot needs. The installer target for GitHub Copilot (epic #482,
items 6 and 9) cannot be decided or built without verified requirements.

Evidence

• docs/bob-install-requirements.md is the model: every claim marked verified /
  documented / unverified with its environment; §1 file tree, §2 <plugin root>
  resolution, §3 symlinks/reload, §4 naming, §5 collisions, §6 missing features.
• <plugin root> is "two levels above this skill's own directory"
  (skills/attunement/SKILL.md:33-36); skills need references/ and the root-level
  scripts/check-public-safety.sh beside skills/ (Bob doc §1).
• scripts/reserved-skill-names.txt:1-15 records per-client built-in names with source and
  retrieval date; scripts/check-skill-shape.sh rule 3 (:205-216) fails when a skill name
  collides with one.

Expected

A docs/ document for GitHub Copilot, linking the agent-neutral core doc (epic item 1), that
answers epic requirement R1 for GitHub Copilot separately for each surface (Copilot CLI and VS Code agent mode): skill directory locations at global and
project scope; whether it reads SKILL.md name/description; whether supporting files
beside SKILL.md are readable; reload behaviour; name-collision precedence; how
<plugin root> is supplied or derived; what <plugin root> is at each scope and whether
its references/ and scripts/ siblings collide with paths users commonly own there;
whether a format translation is needed; and which adept features have no equivalent.

Per R2, the same PR adds GitHub Copilot's documented built-in command names to
scripts/reserved-skill-names.txt with source URL and retrieval date. If any collides with
an adept skill name, stop: the collision is filed as its own rename issue, not shipped here.

A Copilot project install's <plugin root> may be .github/, where .github/scripts/ is a
common user path (this repository has .github/scripts/check-records.sh); record it.

Proposed approach

Read GitHub Copilot's current documentation, test against a live install where available, mark
each claim, and write the doc without restating facts the core doc owns. Items 2–5 of the
epic all edit scripts/reserved-skill-names.txt, so they merge one at a time. Bump
.claude-plugin/plugin.json (PATCH).

Part of #482
Blocked by #483