Add extensible CLI commands (#853)

* Add extensible CLI commands

* Handle plugin SystemExit failures

docs/custom_commands.md

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+# Custom CLI Commands
+
+MVT can load additional top-level commands into `mvt-ios` and `mvt-android`.
+Custom commands are different from [custom forensic modules](development.md#custom-modules):
+commands add new CLI operations, while modules add analysis steps to existing
+`check-*` commands.
+
+!!! warning
+
+    Custom commands run as trusted Python code inside the MVT process. Install
+    or load commands only from sources you trust. MVT does not sandbox
+    third-party commands, and the MVT maintainers do not maintain them.
+
+## Install a Command Package
+
+Python packages can register a Click command or group for either MVT CLI. A
+minimal package can expose this command from `my_mvt_plugin.py`:
+
+```python
+import click
+
+
+@click.command()
+@click.argument("path", type=click.Path(exists=True))
+def summarize(path):
+    """Summarize an acquisition."""
+    click.echo(f"Summarizing {path}")
+```
+
+Register the object in the package's `pyproject.toml`. The entry-point name is
+the command users invoke:
+
+```toml
+[project.entry-points."mvt.ios.cli_plugins"]
+summarize = "my_mvt_plugin:summarize"
+
+[project.entry-points."mvt.android.cli_plugins"]
+summarize = "my_mvt_plugin:summarize"
+```
+
+Use only the iOS or Android group if the command is platform-specific. After
+installing the package in the same environment as MVT, it appears directly in
+the appropriate CLI:
+
+```bash
+mvt-ios summarize ./ios-backup
+mvt-android summarize ./androidqf-output
+```
+
+For a `pipx` installation of MVT, inject the plugin into MVT's environment:
+
+```bash
+pipx inject mvt my-mvt-plugin
+```
+
+When MVT is installed in an active virtual environment, install the plugin with
+`pip` in that environment.
+
+## Load a Command File
+
+For local commands that are not packaged, create a Python file that exports one
+Click command or group named `cli`:
+
+```python
+import click
+
+
+@click.command("case-summary")
+@click.argument("path", type=click.Path(exists=True))
+def cli(path):
+    """Summarize a case directory."""
+    click.echo(f"Summarizing {path}")
+```
+
+Pass the file before the custom command name:
+
+```bash
+mvt-ios --load-command ./case_summary.py case-summary ./ios-backup
+```
+
+`--load-command` can be repeated and also accepts a folder. MVT loads
+non-hidden top-level `*.py` files in sorted order and skips `__init__.py`.
+Every loaded file must export one `cli` object.
+
+To load a file or folder on every invocation, set the platform-specific
+environment variable:
+
+```bash
+export MVT_IOS_CUSTOM_COMMANDS=./ios_commands
+export MVT_ANDROID_CUSTOM_COMMANDS=./android_commands
+```
+
+## Naming and Errors
+
+Built-in MVT commands cannot be replaced. External command names must also be
+unique; when installed packages or environment paths collide, MVT keeps the
+first command and logs a warning. A collision from an explicit
+`--load-command` is a usage error.
+
+A package entry point or environment command that cannot be imported appears
+as a marked broken command without preventing other MVT commands from working.
+Invoke that command to see its package or file source and the underlying error.
+An invalid command supplied explicitly with `--load-command` fails immediately
+with a usage error.
+
+Installed command packages use the entry-point name as the CLI command name.
+The entry point must resolve to a `click.Command` or `click.Group`.