fix: pair-agent skill compliance + fix all 16 pre-existing test failures

Root cause: pair-agent was added without completing the gen-skill-docs
compliance checklist. All 16 failures traced back to this.

Fixes:
- Sync package.json version to VERSION (0.15.9.0)
- Add "(gstack)" to pair-agent description for discoverability
- Add pair-agent to Codex path exception (legitimately documents ~/.codex/)
- Add CLI_COMMANDS (status, pair-agent, tunnel) to skill parser allowlist
- Regenerate SKILL.md for all hosts (claude, codex, factory, kiro, etc.)
- Update golden file baselines for ship skill
- Fix relink tests: pass GSTACK_INSTALL_DIR to auto-relink calls so they
  use the fast mock install instead of scanning real ~/.claude/skills/gstack

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "gstack",
-  "version": "0.15.8.0",
+  "version": "0.15.9.0",
   "description": "Garry's Stack — Claude Code skills + fast headless browser. One repo, one install, entire AI engineering workflow.",
   "license": "MIT",
   "type": "module",