chore: add shared skills catalog (19 skills), installers, manifest, validator

skills/grill-me/SKILL.md

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+---
+name: grill-me
+description: Use when the user wants to stress-test an architecture plan, interrogate a design, or explicitly says "grill me". Relentlessly stress-tests technical plans before implementation starts.
+---
+
+# Grill Me — Architecture & Plan Stress Testing
+
+## Purpose
+Prevent implementation mistakes by rigorously interviewing the developer about their design, assumptions, failure modes, data contracts, and scaling considerations before any code is written.
+
+## Rules of Engagement
+1. **One or Two Questions at a Time:** Do not dump a checklist of 10 questions. Ask 1-2 pointed, high-impact questions per turn to keep the discussion focused.
+2. **Challenge Assumptions:** Look for hidden single points of failure, unstated latency/concurrency assumptions, state synchronization flaws, and unbounded growth.
+3. **Explore Failure Modes:**
+   - What happens when network calls time out or third-party APIs fail?
+   - How does the system recover from crash/reboot mid-operation?
+   - What happens during concurrent writes or race conditions?
+4. **Demand Concrete Boundaries:**
+   - Invariants and preconditions.
+   - Exact schema and error codes, not vague types.
+   - Zero-downtime migration and rollback paths.
+5. **No Code Until Approved:** Refuse to jump to implementation until the plan is decision-complete and stress-tested.
+6. **Closing the Grill:** When all material ambiguities and risks are addressed, summarize the verified architecture plan and ask for confirmation to proceed.