feat: proactive skill suggestions + opt-out + trigger phrase tests

- Preamble reads proactive config via gstack-config
- Root SKILL.md.tmpl has lifecycle map (stage → skill suggestion)
- Users can opt out ("stop suggesting") / opt in ("be proactive again")
- Restored trigger phrase validation tests (16 skills × "Use when" check)
- Added missing "Use when" trigger phrases to /debug and /office-hours

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

debug/SKILL.md.tmpl

@@ -4,6 +4,8 @@ version: 1.0.0
 description: |
   Systematic debugging with root cause investigation. Four phases: investigate,
   analyze, hypothesize, implement. Iron Law: no fixes without root cause.
+  Use when asked to "debug this", "fix this bug", "why is this broken",
+  "investigate this error", or "root cause analysis".
 allowed-tools:
   - Bash
   - Read