docs: add agent guidance files and templates

.claude/skills/review-diff/SKILL.md

@@ -0,0 +1,56 @@
+---
+description: Review the current git diff against the task brief and approved plan in a fresh read-only context. Use before finalizing Standard or Cautious tasks.
+disable-model-invocation: true
+context: fork
+disallowed-tools: Write Edit MultiEdit
+---
+
+Review the current diff against the task brief and approved plan.
+
+Use the live git state below as the source of truth.
+
+## Current branch
+
+!`git branch --show-current || true`
+
+## Git status
+
+!`git status --short || true`
+
+## Diff stat
+
+!`git diff --stat HEAD || true`
+
+## Diff
+
+!`git diff HEAD || true`
+
+## Instructions
+
+Look for:
+
+- missing requirements;
+- bugs;
+- missing edge cases;
+- missing tests;
+- regressions;
+- security risks;
+- data consistency risks;
+- concurrency or idempotency risks;
+- unrelated changes.
+
+Do not comment on subjective style preferences unless they affect correctness or maintainability.
+
+Return exactly:
+
+## Blockers
+
+## Warnings
+
+## Suggested tests
+
+## Out-of-scope changes
+
+## Verdict
+
+Use `approve` or `request changes` for the verdict.