Add installable create-plan skills for codex, claude-code, and opencode

skills/create-plan/codex/templates/milestone-plan.md

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+# [Plan Title]
+
+## Overview
+- **Goal:** [One sentence describing the end state]
+- **Created:** YYYY-MM-DD
+- **Status:** In Progress | Complete
+
+## Context
+
+### Requirements
+[Gathered requirements from user questions]
+
+### Constraints
+[Technical, business, or timeline constraints]
+
+### Success Criteria
+[How we know this is complete]
+
+## Architecture
+
+### Design Decisions
+[Key architectural choices and rationale]
+
+### Component Relationships
+[How pieces fit together]
+
+### Data Flow
+[How data moves through the system]
+
+## Milestones
+
+### M1: [Name]
+**Description:** [What this milestone achieves]
+
+**Acceptance Criteria:**
+- [ ] [Criterion 1]
+- [ ] [Criterion 2]
+
+**Stories:** S-101, S-102, S-103...
+
+**Milestone Completion Rule (MANDATORY):**
+- Run lint/typecheck/tests for changed files.
+- Commit locally (DO NOT push).
+- Stop and ask user for feedback.
+- Apply feedback, re-check changed files, commit again.
+- Move to next milestone only after user approval.
+
+---
+
+### M2: [Name]
+**Description:** [What this milestone achieves]
+
+**Acceptance Criteria:**
+- [ ] [Criterion 1]
+- [ ] [Criterion 2]
+
+**Stories:** S-201, S-202, S-203...
+
+**Milestone Completion Rule (MANDATORY):**
+- Run lint/typecheck/tests for changed files.
+- Commit locally (DO NOT push).
+- Stop and ask user for feedback.
+- Apply feedback, re-check changed files, commit again.
+- Move to next milestone only after user approval.
+
+---
+
+## Technical Specifications
+
+### Types & Interfaces
+```typescript
+// Key type definitions
+```
+
+### API Contracts
+```typescript
+// Endpoint signatures, request/response shapes
+```
+
+### Constants & Enums
+```typescript
+// Shared constants
+```
+
+## Files Inventory
+
+| File | Purpose | Milestone |
+|------|---------|-----------|
+| `path/to/file.ts` | [What it does] | M1 |
+| `path/to/other.ts` | [What it does] | M2 |
+
+---
+
+## Related Plan Files
+
+This file is part of the plan folder under `docs/plans/` (or `docs/plan/` if that repo convention is used):
+- `original-plan.md` - Original approved plan (reference for original intent)
+- `final-transcript.md` - Final planning transcript (reference for rationale/context)
+- `milestone-plan.md` - This file (full specification)
+- `story-tracker.md` - Status tracking (must be kept up to date)
+- `continuation-runbook.md` - Resume/execution context (read first)