Feature: Add /init-template skill for first-time repo setup

[striderZA]

Summary

Create an /init-template skill that transforms a cloned OCGS template into a clean, ready-to-use project for a new game.

Problem

When users clone the template, they inherit:

• Our marketing README (release history, PayPal links, v0.2.0 changelog)
• Internal docs (UPGRADING.md, session logs, release notes)
• Our commit history and git tags
• Stale design/ and src/ content
• Our model mappings in AGENTS.md

Users must manually clean all of this before starting their own game.

Proposed Changes

New skill: /init-template

An interactive skill that:

1. Prompt for project info — game name, engine, genre, team size
2. Replace README.md — swap marketing README with a clean project template
3. Update AGENTS.md — replace internal model mappings with user's preferred models
4. Update opencode.json — clean internal plugin paths, set project name
5. Remove internal files — UPGRADING.md, release notes, internal docs
6. Clear design/ and src/ — remove existing GDDs and source files
7. Optional: reset git history — offer to squash to a single fresh commit

Acceptance Criteria

• /init-template skill created and functional
• README.md replaced with blank project template
• AGENTS.md updated with user's engine/model choices
• Internal docs removed from template
• design/ and src/ cleared to empty state
• Git history reset option provided
• Documentation updated (quick-start.md, skills-reference.md)