feat: add reviewed planning skill

create-plan/assets/implementation-plan-template.md

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+# Implementation Plan
+
+> **Status:** Draft
+
+## Outcome and boundaries
+
+- **Problem and target:**
+- **In scope:**
+- **Out of scope:**
+- **Approach:**
+
+## Key files, evidence, and decisions
+
+| File or source | Why it matters | Decision or plan impact |
+|---|---|---|
+
+- **Open gate:** Omit this line when no gate remains.
+
+## Tasks
+
+#### T1 — First implementation task
+
+- **Change:**
+  - One concrete edit or behavior per bullet.
+- **Starts at:** Non-exhaustive paths and symbols.
+- **Depends on:** Omit when independent.
+- **Tests:** Name the behavior each test protects.
+- **Verify:**
+  - Run an exact command; state its expected successful signal.
+- **Risk/recovery:** Omit when immaterial.
+
+## Final acceptance
+
+- **Checks:** Repository gates and end-to-end behavior checks.
+- **End state:** Observable implementation outcome.
+- **Deferrals or blockers:** Omit when none.