[WIP] Add-extension skill

.claude/skills/add-extension/.gitignore

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

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

@@ -0,0 +1,154 @@
+# /add-extension - SE-2 Extension Merger
+
+Claude Code skill for adding Scaffold-ETH-2 extensions to existing projects post-creation.
+
+## Quick Start
+
+```bash
+# In SE-2 project root
+/add-extension erc-20
+
+# Preview changes first
+/add-extension subgraph --dry-run
+
+# Skip confirmation prompts
+/add-extension ponder --yes
+```
+
+## Options
+
+| Flag | Short | Description |
+|------|-------|-------------|
+| `--list` | `-L` | List all available extensions with repo info |
+| `--dry-run` | `-d` | Preview changes without applying |
+| `--force` | `-f` | Reinstall even if already installed |
+| `--yes` | `-y` | Skip confirmation prompts |
+| `--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 (repository URL, branch) from [create-eth](https://github.com/scaffold-eth/create-eth/blob/main/src/extensions/index.ts)
+3. **Fetch** - Clones extension branch from the repository defined in registry
+4. **Framework** - Auto-detects hardhat/foundry from project
+5. **Analyze** - Compares extension files vs project
+6. **Merge** - Copies new files, merges package.json, prompts for conflicts
+7. **Track** - Records extension in `package.json` under `scaffoldEth.extensions`
+
+### Framework Handling
+
+| Project State | Behavior |
+|---------------|----------|
+| Has Hardhat | Uses Hardhat files automatically |
+| Has Foundry | Uses Foundry files automatically |
+| Has neither | Prompts or uses `-s` flag |
+| Flag mismatch | Errors (won't install wrong framework) |
+
+### File Handling
+
+| File Type | Action |
+|-----------|--------|
+| New files | Copied directly |
+| package.json | Auto-merged (deps, scripts) |
+| `.args.mjs` files | Template-based merge |
+| Other conflicts | Manual merge (Claude assists) |
+
+## Extension Tracking
+
+Installed extensions tracked in `package.json`:
+
+```json
+{
+  "scaffoldEth": {
+    "extensions": ["erc-20", "subgraph"]
+  }
+}
+```
+
+## Architecture
+
+```
+.claude/skills/add-extension/
+├── skill.mjs              # Entry point & CLI
+├── SKILL.md               # Claude skill definition
+├── README.md              # This file
+└── lib/
+    ├── constants.mjs      # Shared constants (URLs, fallbacks)
+    ├── templateUtils.mjs  # Shared utilities
+    ├── validator.mjs      # SE-2 detection & extension validation
+    ├── fetcher.mjs        # GitHub/local extension fetching
+    ├── analyzer.mjs       # File diff & framework detection
+    ├── merger.mjs         # File operations & package.json merge
+    ├── extensionTracker.mjs  # Extension tracking & args loading
+    ├── templateMerger.mjs # Template-based file generation
+    └── create-eth-utils/  # Utils copied from create-eth
+```
+
+## Example Output
+
+```
+🏗️  Scaffold-ETH-2 Extension Merger
+Adding extension: erc-20
+
+✓ Detected SE-2 project (hardhat)
+✓ Fetched extension (23 files)
+
+Extension Changes Summary:
+
+New files (5):
+  + packages/nextjs/app/erc20/page.tsx
+  + packages/hardhat/contracts/ERC20Token.sol
+
+package.json changes:
+  Dependencies:
+    + @openzeppelin/contracts@^5.0.0
+  Scripts:
+    + deploy:erc20
+
+📝 Applying changes...
+✓ Tracked extension in package.json
+
+==================================================
+Extension "erc-20" merge complete!
+==================================================
+
+✓ Applied: 6 files
+
+Next steps:
+  1. Run yarn install
+  2. Test your application
+  3. Commit changes
+```
+
+## 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 (defines repo+branch for each extension) |
+| [create-eth-extensions](https://github.com/scaffold-eth/create-eth-extensions) | Default extension source (each extension = git branch) |
+
+Extensions are fetched from whatever repository is defined in the registry. Currently all point to `create-eth-extensions`, but the system supports any repository URL.
+
+## Requirements
+
+- SE-2 project root directory
+- Git installed
+- Yarn (not npm)

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

@@ -0,0 +1,54 @@
+---
+name: add-extension
+description: Merge a scaffold-eth extension into the current project
+disable-model-invocation: false
+allowed-tools: Bash, Read
+user-invocable: true
+---
+
+Merge the `$0` extension into this Scaffold-ETH-2 project.
+
+## Execution
+
+Run the merger script:
+```bash
+node .claude/skills/add-extension/skill.mjs $0 $@
+```
+
+## Post-Merge Tasks
+
+1. Review the output for any skipped files needing manual merge
+2. If manual merges needed, help user resolve conflicts
+3. Remind user to run `yarn install` if dependencies changed
+4. Summarize what was added/modified
+
+## Options
+
+| Flag | Description |
+|------|-------------|
+| `--dry-run`, `-d` | Preview only |
+| `--force`, `-f` | Reinstall if installed |
+| `--yes`, `-y` | Skip prompts |
+| `--verbose`, `-v` | Detailed errors |
+| `--local`, `-l` | Local extension path |
+| `-s`, `--solidity-framework` | Choose hardhat/foundry |
+
+## Framework Selection
+
+- Project has hardhat → uses hardhat files (no prompt)
+- Project has foundry → uses foundry files (no prompt)
+- Project has neither → prompts or uses `-s` flag
+- Flag mismatch with project → errors
+
+## Notes
+
+- Temp files are in `.claude/skills/add-extension/.tmp`, not `/tmp`
+- Don't ask permission for `.tmp` folder operations
+
+## Examples
+
+```bash
+/add-extension erc-20
+/add-extension subgraph --dry-run
+/add-extension ponder -s hardhat --yes
+```