feat: add hybrid /add-extension skill for merging SE-2 extensions

Split extension merging into deterministic (Node.js script) and intelligent
(Claude AI) operations. The script handles fetch, copy, and package.json
merge, then outputs structured JSON tasks for Claude to process .args.mjs
files and resolve file conflicts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: technophile-04

.claude/skills/add-extension/.gitignore

@@ -0,0 +1 @@
+.tmp

.claude/skills/add-extension/README.md

@@ -0,0 +1,86 @@
+# /add-extension - SE-2 Extension Merger (Hybrid)
+
+Claude Code skill for adding Scaffold-ETH-2 extensions to existing projects post-creation.
+
+Uses a **hybrid approach**: a Node.js script handles deterministic operations (fetch, copy, package.json merge), while Claude handles intelligent file merging (.args.mjs processing).
+
+**Requirements:** SE-2 project root, Git, Yarn
+
+## Quick Start
+
+```bash
+# In SE-2 project root
+/add-extension erc-20
+
+# Preview changes first
+/add-extension subgraph --dry-run
+
+# Choose framework explicitly
+/add-extension ponder -s hardhat
+```
+
+## Options
+
+| Flag | Short | Description |
+|------|-------|-------------|
+| `--list` | `-L` | List all available extensions with repo info |
+| `--dry-run` | `-d` | Preview changes without applying |
+| `--verbose` | `-v` | Show detailed error messages |
+| `--local <path>` | `-l` | Use local extension repo path |
+| `--solidity-framework <name>` | `-s` | Choose hardhat/foundry (only if project has neither) |
+
+## Available Extensions
+
+Fetched dynamically from [create-eth](https://github.com/scaffold-eth/create-eth/blob/main/src/extensions/create-eth-extensions.ts).
+
+Run `/add-extension --list` for current list with repository details.
+
+## How It Works
+
+1. **Validate** - Checks for SE-2 project (scaffold.config.ts)
+2. **Registry** - Fetches extension config from create-eth registry
+3. **Fetch** - Clones extension branch via git
+4. **Framework** - Auto-detects hardhat/foundry from project
+5. **Analyze** - Compares extension files vs project, reads .args.mjs as raw text
+6. **Deterministic merge** - Script copies new files, merges package.json, registers workspaces
+7. **AI merge** - Script outputs structured JSON tasks; Claude reads target files and applies .args.mjs exports intelligently
+
+### File Handling
+
+| File Type | Handler | Action |
+|-----------|---------|--------|
+| New files | Script | Copied directly |
+| package.json | Script | Auto-merged (deps, scripts) |
+| Workspaces | Script | Registered in root package.json |
+| `.args.mjs` files | Claude | Reads exports, modifies target files |
+| `.template.mjs` files | Claude | Extracts content, writes/merges target |
+| File conflicts | Claude | Reads both versions, merges intelligently |
+
+## Architecture
+
+```
+SKILL.md           - Skill instructions (comprehensive .args.mjs guide)
+skill.mjs          - CLI entry point, orchestration, JSON output
+lib/validator.mjs  - SE-2 detection, registry fetching
+lib/fetcher.mjs    - Git clone / local path handling
+lib/analyzer.mjs   - File categorization, .args.mjs raw reading
+lib/merger.mjs     - Deterministic operations (copy, package.json merge)
+lib/constants.mjs  - Registry URLs, fallback extensions
+```
+
+## Development
+
+```bash
+# Test locally with dry-run
+node .claude/skills/add-extension/skill.mjs erc-20 --dry-run
+
+# Test with local extension repo
+/add-extension erc-721 --local ../create-eth-extensions --dry-run
+```
+
+## Source Repositories
+
+| Repo | Purpose |
+|------|---------|
+| [create-eth](https://github.com/scaffold-eth/create-eth) | SE-2 CLI, extension registry |
+| [create-eth-extensions](https://github.com/scaffold-eth/create-eth-extensions) | Default extension source (each extension = git branch) |