Add phase-gated-debugging skill (#409)

* Add phase-gated-debugging skill

* Fix risk label: low -> safe (valid value per validate_skills.py)

skills/phase-gated-debugging/SKILL.md

@@ -0,0 +1,61 @@
+---
+name: phase-gated-debugging
+description: "Use when debugging any bug. Enforces a 5-phase protocol where code edits are blocked until root cause is confirmed. Prevents premature fix attempts."
+risk: safe
+source: community
+date_added: "2026-03-28"
+---
+
+# Phase-Gated Debugging
+
+## Overview
+
+AI coding agents see an error and immediately edit code. They guess at fixes, get it wrong, and spiral. This skill enforces a strict 5-phase protocol where you CANNOT edit source code until the root cause is identified and confirmed.
+
+Based on [claude-debug](https://github.com/krabat-l/claude-debug) (full plugin with PreToolUse hook enforcement).
+
+## The Protocol
+
+### Phase 1: REPRODUCE
+Run the failing command/test. Capture the exact error. Run 2-3 times for consistency.
+- Do NOT read source code
+- Do NOT hypothesize
+- Do NOT edit any files
+
+### Phase 2: ISOLATE
+Read code. Add diagnostic logging marked `// DEBUG`. Re-run with diagnostics. Binary search to narrow down.
+- Only `// DEBUG` marked logging is allowed
+- Do NOT fix the bug even if you see it
+
+### Phase 3: ROOT CAUSE
+Analyze WHY at the isolated location. Use "5 Whys" technique. Remove debug logging.
+
+State: "This is my root cause analysis: [explanation]. Do you agree, or should I investigate further?"
+
+**WAIT for user confirmation. Do NOT proceed without it.**
+
+### Phase 4: FIX
+Remove all `// DEBUG` lines. Apply minimal change addressing confirmed root cause.
+- Only edit files related to root cause
+- Do NOT refactor unrelated code
+
+### Phase 5: VERIFY
+Run original failing test — must pass. Run related tests. For intermittent bugs, run 5+ times.
+If verification fails: root cause was wrong, go back to Phase 2.
+
+## Bug-Type Strategies
+
+| Type | Technique |
+|------|-----------|
+| Crash/Panic | Stack trace backward — trace the bad value to its source |
+| Wrong Output | Binary search — log midpoint, halve search space each iteration |
+| Intermittent | Compare passing vs failing run logs — find ordering divergence |
+| Regression | `git bisect` — find the offending commit |
+| Performance | Timing at stage boundaries — find the bottleneck |
+
+## Key Rules
+
+1. NEVER edit source code in phases 1-3 (except `// DEBUG` in phase 2)
+2. NEVER proceed past phase 3 without user confirmation
+3. ALWAYS reproduce before investigating
+4. ALWAYS verify after fixing