fix(#812): honor COPILOT_HOME in Copilot global config-dir resolution (#814)

* fix(#812): honor COPILOT_HOME in Copilot global config-dir resolution

getGlobalConfigDir('copilot') resolved the global config directory using
only --config-dir > COPILOT_CONFIG_DIR > ~/.copilot, ignoring the
COPILOT_HOME env var. Per GitHub's Copilot CLI docs, COPILOT_HOME
overrides the default ~/.copilot location (and user-level hooks are read
from $COPILOT_HOME/hooks/), so a global --copilot install wrote all
artifacts (skills, agents, copilot-instructions.md, the gsd-session.json
hook) to ~/.copilot even when the user relocated their Copilot home,
making them undiscoverable by Copilot CLI.

Mirror the codex/CODEX_HOME branch: precedence is now
--config-dir > COPILOT_CONFIG_DIR > COPILOT_HOME > ~/.copilot. Uninstall
uses the same resolver, so it stays symmetric.

Also: document COPILOT_HOME in the installer --help notes, the
USER-GUIDE env-var table, and the installer-migrations Copilot row; and
clear COPILOT_HOME in the two default-path test suites so they stay
hermetic now that the resolver honors it.

Closes #812

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* chore(#812): add changeset for PR #814

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

docs/USER-GUIDE.md

@@ -749,7 +749,7 @@ WINDSURF_CONFIG_DIR=~/.codeium/windsurf-next npx @opengsd/gsd-core@latest --wind
 | Gemini CLI | `~/.gemini` | `GEMINI_CONFIG_DIR` |
 | OpenCode | `XDG_CONFIG_HOME/opencode` | `OPENCODE_CONFIG_DIR` |
 | Codex | (per Codex CLI) | `--config-dir` flag |
-| Copilot | `~/.copilot` | `COPILOT_CONFIG_DIR` |
+| Copilot | `~/.copilot` | `COPILOT_CONFIG_DIR` (or `COPILOT_HOME`) |
 | Cursor | `~/.cursor` | `CURSOR_CONFIG_DIR` |
 | Windsurf | `~/.codeium/windsurf` | `WINDSURF_CONFIG_DIR` |
 | Antigravity | auto-detected | `ANTIGRAVITY_CONFIG_DIR` |