Move install instructions into per-skill docs and remove single INSTALL doc

docs/CREATE-PLAN.md

@@ -4,9 +4,55 @@
 
 Create structured implementation plans with milestone and story tracking.
 
+## Requirements
+
+- Superpowers skills installed from: https://github.com/obra/superpowers
+- Required dependencies:
+  - `brainstorming`
+  - `writing-plans`
+
+If dependencies are missing, stop and return:
+
+"Missing dependency: Superpowers planning skills are required (`brainstorming`, `writing-plans`). Install from https://github.com/obra/superpowers, then retry."
+
+## Install
+
+### Codex
+
+```bash
+mkdir -p ~/.codex/skills/create-plan
+cp -R skills/create-plan/codex/* ~/.codex/skills/create-plan/
+```
+
+### Claude Code
+
+```bash
+mkdir -p ~/.claude/skills/create-plan
+cp -R skills/create-plan/claude-code/* ~/.claude/skills/create-plan/
+```
+
+### OpenCode
+
+```bash
+mkdir -p ~/.opencode/skills/create-plan
+cp -R skills/create-plan/opencode/* ~/.opencode/skills/create-plan/
+```
+
+## Verify Installation
+
+```bash
+test -f ~/.codex/skills/create-plan/SKILL.md || true
+test -f ~/.claude/skills/create-plan/SKILL.md || true
+test -f ~/.opencode/skills/create-plan/SKILL.md || true
+```
+
+Verify Superpowers dependencies exist in your agent skills root:
+
+- `.../skills/brainstorming/SKILL.md`
+- `.../skills/writing-plans/SKILL.md`
+
 ## Key Behavior
 
-- Requires Superpowers dependencies (`brainstorming`, `writing-plans`).
 - Creates plans under `ai_plan/YYYY-MM-DD-<short-title>/`.
 - Ensures `/ai_plan/` is in `.gitignore`.
 - Commits `.gitignore` update locally when added.