feat: Sandbox implementation with seatbelt

sandbox/profiles/README.md

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+# PMG Sandbox Profiles
+
+This directory contains built-in sandbox policies for PMG package managers.
+
+## Available Profiles
+
+### npm-restrictive
+
+Restrictive policy for the npm ecosystem (npm, pnpm, yarn, bun).
+
+**Features:**
+- Allows read access to current directory, package manager configs, and caches
+- Restricts write access to `node_modules/` and lockfiles only
+- Blocks access to sensitive files (`~/.ssh`, `~/.aws`, `.env` files)
+- Allows network access to npm registries only
+- Permits Node.js and git execution, blocks shell and curl/wget
+
+**Use when:** You want balanced protection for npm package installations
+
+### pypi-restrictive
+
+Restrictive policy for the PyPI ecosystem (pip, pip3, poetry, uv).
+
+**Features:**
+- Allows read access to current directory, pip configs, and caches
+- Restricts write access to virtual environments and package caches
+- Blocks access to sensitive files (`~/.ssh`, `~/.aws`, `.env` files)
+- Allows network access to PyPI registries only
+- Permits Python, compilers (for native extensions), and git
+
+**Use when:** You want balanced protection for pip package installations
+
+## Custom Policies
+
+You can create custom sandbox policies by:
+
+1. Copying one of the built-in profiles
+2. Modifying the rules to suit your needs
+3. Referencing the custom profile in your PMG config:
+
+```yaml
+sandbox:
+  enabled: true
+  policies:
+    npm:
+      enabled: true
+      profile: /path/to/custom-npm-policy.yml
+```
+
+## Policy Schema
+
+See the [Policy Schema Documentation](../policy.go) for details on the YAML structure.
+
+### Supported Variables
+
+- `${HOME}`: User home directory
+- `${CWD}`: Current working directory
+- `${PM_CACHE}`: Package manager cache directory
+- `${TMPDIR}`: Temporary directory
+
+### Violation Modes
+
+- `block`: Block execution on policy violation (recommended)
+- `warn`: Log warning but allow execution
+- `allow`: Allow all operations (disables sandbox)