feat(skills): add systematic-debugging — 6-phase structured debugging

[JYHTHEGREAT]

Context

Contribution from jyh-system. The most common AI coding failure mode is "guess a fix → doesn't work → guess another → repeat". This skill structurally prevents that.

What This Adds

systematic-debugging skill — a 6-phase methodology that enforces root cause investigation before any fix.

The 6 Phases

1. Quick Assessment — classify error, external research (5 min), branch decision
2. Root Cause Investigation — read errors carefully, reproduce, check changes, trace data flow
3. Pattern Analysis — find working similar code, compare, identify differences
4. Hypothesis & Testing — single hypothesis, minimal test, clean state required
5. Implementation — failing test first, single fix, verify
6. Architecture Question — after 3 failed fixes, stop and escalate

Iron Law

Never propose a fix without root cause investigation.

Key Differentiators vs existing /debugging-strategies

• External research mandatory (Phase 0) — don't rely on LLM self-knowledge
• Fix counter — tracks attempts, auto-escalates at 3 failures
• Clean state enforcement — no cumulative fixes, rollback between hypotheses
• Red Flags checklist — catches common rationalizations ("let me just quickly fix this")

Supporting References

• external-research-guide.md — structured lookup in official docs, GitHub Issues, Stack Overflow
• root-cause-tracing.md — 5-step call stack backtracing methodology

Changes

• skills/systematic-debugging/SKILL.md (185 lines)
• skills/systematic-debugging/references/external-research-guide.md (68 lines)
• skills/systematic-debugging/references/root-cause-tracing.md (73 lines)

Existing Code Impact

None. Pure addition. Can coexist with existing /debugging-strategies or replace it.

Test Plan

• Trigger with an intentional build error → should run Phase 0 (classify + external research) before suggesting any fix
• Simulate 3 failed fixes → should escalate to Phase 4.5 Architecture Question
• Verify Red Flags detection when Claude tries to skip investigation

🤖 Generated with Claude Code