Harden AI agent rules and compile learning workflow

docs/00-governance/common_failures.md

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+# Common Compile Failures
+**Last Updated:** 2026-04-05
+
+Record concise error fingerprints and verified fixes.
+
+## Entry Template
+- **Date:** YYYY-MM-DD
+- **Error:** `CSXXXX`
+- **Fingerprint:** file + line + short symptom
+- **Root cause:** one sentence
+- **Fix:** one to three concrete steps
+- **Prevention link:** guardrail/pattern doc entry added
+
+## Starter Examples
+1. **`CS0115` no suitable method found to override**
+   - Root cause: incorrect NT8 override signature.
+   - Fix: replace with exact official signature; recompile.
+
+2. **`CS0507` cannot change access modifiers when overriding**
+   - Root cause: used `public override`/`private override` instead of `protected override`.
+   - Fix: change to `protected override`; recompile.
+
+3. **`CS0246` type or namespace not found**
+   - Root cause: missing using/namespace or undefined domain type.
+   - Fix: add correct namespace reference or confirmed type definition.
+
+4. **`CS1503` argument type mismatch**
+   - Root cause: wrong overload or argument ordering.
+   - Fix: align call with exact method signature and parameter types.
+
+5. **C# 6+ syntax in C# 5 project**
+   - Root cause: use of `$""`, `?.`, `nameof`, expression-bodied members, etc.
+   - Fix: rewrite using C# 5 compatible constructs.