Feature: Custom Python Commands/Plugins System

[genro]

Problem

Currently, when you need to execute custom logic in gtext templates, you have to:

1. Write inline shell commands with Python (python -c "...")
2. Deal with multi-line quoting issues
3. Configure security rules for each command pattern
4. No easy way to reuse logic across projects

Example that's hard to write:

```include
cli: python -c "
import tomli
from pathlib import Path
# ... complex logic with escaping issues
"

## Proposed Solution

Add a **plugin/custom commands system** that allows registering Python scripts or functions as named commands.

### Option 1: Script-based Commands

Configure custom commands in `.gtext/config.yml` or `pyproject.toml`:

```toml
[tool.gtext.commands]
scandir = "scripts/scandir.py"
listprojects = "scripts/list_projects.py"

Usage in templates:

```include
scandir: .

### Option 2: Plugin Class-based

More powerful approach with proper API:

```python
# .gtext/plugins/scandir.py
from gtext.plugins import Plugin

class ScandirPlugin(Plugin):
    name = "scandir"
    description = "Scan directory for Python projects"
    
    def execute(self, path=".", **kwargs):
        # Implementation
        return markdown_output

Usage:

```include
scandir: .

```include
listprojects: --format table

## Benefits

- ✅ **Cleaner templates** - No complex shell escaping
- ✅ **Reusable** - Share plugins across projects
- ✅ **Type-safe** - Python functions instead of shell strings
- ✅ **Testable** - Easy to unit test plugins
- ✅ **Security** - Plugin allowlist instead of shell command patterns
- ✅ **Documentation** - Plugins can self-document
- ✅ **IDE support** - Autocomplete for plugin names

## Real-World Use Case

**We-Birds README.md.gtext** needs to dynamically list all tools by scanning subdirectories and reading their `pyproject.toml` files. Currently impossible to do cleanly with inline commands.

With plugins:
```markdown
## Tools in the Flock 🪶

```include
scandir: . --type python-projects --format markdown

## Implementation Ideas

1. **Discovery**: Load plugins from `.gtext/plugins/` or configured paths
2. **Registration**: Register as new protocol handlers (like `cli:`, `glob:`)
3. **Security**: Plugins are local files, safer than arbitrary shell
4. **Standard library**: Ship common plugins (file operations, git info, etc.)

## Related

Related to #6 (command system refactoring) - could be part of that work.

## Priority

Medium - Would significantly improve DX for complex use cases.