Initial commit with dev backbone template

SKILLS/30-implementation-rules.md

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+# Implementation Rules
+
+## Change policy
+- Prefer edits over rewrites.
+- Keep changes localized.
+- One change = one purpose.
+- Avoid unnecessary abstraction.
+
+## Dependency policy
+- Default: do not add dependencies.
+- If adding: explain why, alternatives considered, and impact.
+
+## Error handling
+- Validate inputs at boundaries.
+- Error messages must be actionable: what failed + what to do next.