feat: add verification and debugging workflow skills

.config/opencode/skills/systematic-debugging/root-cause-tracing.md

@@ -0,0 +1,66 @@
+---
+title: root-cause-tracing
+type: note
+permalink: opencode-config/skills/systematic-debugging/root-cause-tracing
+---
+
+# Root-Cause Tracing
+
+## Overview
+
+Many bugs appear deep in a stack trace, but the origin is often earlier in the call chain.
+
+**Core principle:** trace backward to the original trigger, then fix at the source.
+
+## When to Use
+
+Use this when:
+- The symptom appears far from where bad input was introduced
+- The call chain spans multiple layers or components
+- You can see failure but cannot yet explain origin
+
+## Tracing Process
+
+1. **Capture the symptom clearly**
+   - Exact error text, stack frame, and context.
+
+2. **Find immediate failure point**
+   - Identify the exact operation that throws or misbehaves.
+
+3. **Walk one frame up**
+   - Determine who called it and with which values.
+
+4. **Repeat until source**
+   - Continue tracing callers and values backward until you find where invalid state/data originated.
+
+5. **Fix at source**
+   - Correct the earliest trigger rather than patching downstream symptoms.
+
+## Instrumentation Tips
+
+When manual tracing is hard, add targeted instrumentation before the risky operation:
+
+```ts
+const stack = new Error().stack;
+console.error('debug context', {
+  input,
+  cwd: process.cwd(),
+  envMode: process.env.NODE_ENV,
+  stack,
+});
+```
+
+Guidelines:
+- Log before failure-prone operations, not after.
+- Include values that influence behavior.
+- Capture stack traces for call-path evidence.
+
+## Common Mistake
+
+**Mistake:** fixing where the error appears because it is visible.
+
+**Better:** trace backward and fix where incorrect state is first introduced.
+
+## Pair with Layered Defenses
+
+After fixing the source, apply layered validation from `defense-in-depth.md` so similar failures are blocked earlier in the future.