Introduce built-in templates for enhanced configuration options, support JSONC format

docs/templates.md

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+# Config Templates
+
+Fence includes built-in config templates for common use cases. Templates are embedded in the binary, so you can use them directly without copying files.
+
+## Using templates
+
+Use the `-t` / `--template` flag to apply a template:
+
+```bash
+# Use a built-in template
+fence -t npm-install npm install
+
+# Wraps Claude Code
+fence -t code -- claude
+
+# List available templates
+fence --list-templates
+```
+
+You can also copy and customize templates from [`internal/templates/`](/internal/templates/).
+
+## Available Templates
+
+| Template | Description |
+|----------|-------------|
+| `code` | Production-ready config for AI coding agents (Claude Code, Codex, Copilot, etc.) |
+| `git-readonly` | Blocks destructive commands like `git push`, `rm -rf`, etc. |
+| `local-dev-server` | Allow binding and localhost outbound; allow writes to workspace/tmp |