Add a plugins command to list installed plugins and check updates

Add a "plugins" command to the platform-neutral mvt command. "plugins
list" shows every installed plugin package with its version, where it was
installed from, how many forensic modules it contributes and which
commands it adds. "plugins check-updates" checks for updates immediately,
without waiting for the automatic check, and prints the command which
upgrades a plugin instead of installing anything.

It lives on mvt only. The packages it lists extend mvt-ios and mvt-android
too, but auditing them is not the job of a command which analyses one
platform, and the two platform CLIs should not carry commands which are
not about an acquisition.

The command is registered as a built-in, before any external command, so
that an installed package cannot replace this audit surface.

docs/development/custom_commands.md

@@ -60,7 +60,8 @@ pipx inject mvt my-mvt-plugin
 ```
 
 When MVT is installed in an active virtual environment, install the plugin with
-`pip` in that environment.
+`pip` in that environment. `mvt plugins list` shows the installed packages and
+the commands they add, see [Managing Plugins](plugins.md).
 
 Command packages that need their own settings, such as an API key, should store
 them in a namespaced [plugin configuration file](plugin_configuration.md)
@@ -69,9 +70,9 @@ rather than in MVT's own `config.yaml`.
 ### Commands on `mvt`
 
 The `mvt` command hosts what belongs to neither platform: `version`,
-`completion` and `download-iocs`. A plugin command which is not about the
-acquisition of one platform, such as one which configures the plugin or
-synchronizes the indicators it uses, belongs there too, in the
+`completion`, `plugins` and `download-iocs`. A plugin command which is not
+about the acquisition of one platform, such as one which configures the plugin
+or synchronizes the indicators it uses, belongs there too, in the
 `mvt.cli_plugins` group:
 
 ```toml

docs/development/index.md

@@ -214,6 +214,9 @@ module came from:
 - When a command runs with an `--output` folder, the `command.log` file
   records one line per module source with the source's version or hash and
   the list of modules loaded from it.
+- `mvt plugins list` lists the installed packages, where each of them
+  was installed from and how many modules it contributes, see
+  [Managing Plugins](plugins.md).
 
 ## Profiling
 

docs/development/plugins.md

@@ -0,0 +1,116 @@
+# Managing Plugins
+
+Plugin packages extend MVT with additional
+[forensic modules](index.md#custom-modules) and
+[CLI commands](custom_commands.md). Because installed packages load
+automatically, `mvt plugins` audits what is installed and checks whether
+updates are available. The command lives on `mvt` only, although the packages
+it lists extend `mvt-ios` and `mvt-android` too.
+
+## List Installed Plugins
+
+```bash
+mvt plugins list
+```
+
+```
+                        Installed MVT plugins
+┏━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━┳━━━━━━━━━━━━━━┳━━━━━━━━━┳━━━━━━━━━━━┓
+┃ Name                ┃ Version ┃ Origin       ┃ Modules ┃ Commands  ┃
+┡━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━╇━━━━━━━━━━━━━━╇━━━━━━━━━╇━━━━━━━━━━━┩
+│ mvt-plugin-example  │ 1.2.0   │ pypi         │       4 │ summarize │
+│ mvt-plugin-research │ 0.1.0   │ git+3f9a1c7d │       2 │ -         │
+│ mvt-plugin-local    │ 0.0.1   │ local        │       1 │ triage    │
+└─────────────────────┴─────────┴──────────────┴─────────┴───────────┘
+```
+
+The origin records where each package was installed from: `pypi` for a package
+installed from a package index, `git+<commit>` for a package installed directly
+from a repository, and `local` for a package installed from a local folder or
+archive rather than from an index, including an editable development install.
+The last two columns show how many forensic modules the package contributes and
+which CLI commands it adds.
+
+A plugin whose modules cannot be imported is listed with `error` in the
+`Modules` column rather than breaking the listing.
+
+## Check for Updates
+
+```bash
+mvt plugins check-updates
+```
+
+```
+Plugin updates available:
+  mvt-plugin-example  1.2.0 → 1.3.0
+    Upgrade with: pip install -U mvt-plugin-example
+
+MVT does not install plugin updates. Run the command above when you decide to
+upgrade.
+```
+
+Packages installed from a package index are compared against the latest release
+published for them. A package which was never published, for example a plugin
+distributed only within an organization, is skipped silently.
+
+!!! note
+
+    Packages shown with the `pypi` origin are compared against
+    [PyPI](https://pypi.org), whichever index they were installed from. A
+    plugin installed from a private index under a name which also exists on
+    PyPI is therefore compared against the unrelated public package of that
+    name. Give plugins published to a private index a name which is not taken
+    on PyPI, and treat an unexpected update suggestion as a reason to check
+    where the package would come from.
+
+!!! warning
+
+    MVT never installs or upgrades a plugin itself, it only prints the command
+    which does. Upgrading a plugin in the middle of an investigation changes
+    the modules producing the results, and a plugin runs as trusted code inside
+    the MVT process, so pulling in a new version is a decision for the analyst
+    to make deliberately and not a side effect of running a check.
+
+## Automatic Update Checks
+
+MVT also reports available plugin updates in the banner printed when a command
+starts:
+
+```
+        MVT - Mobile Verification Toolkit
+
+        https://mvt.re
+        Version: 2026.7.29
+
+        Plugin updates available:
+          mvt-plugin-example  1.2.0 → 1.3.0   (pip install -U mvt-plugin-example)
+```
+
+This check runs at most once every 12 hours. In between checks MVT prints the
+findings of the latest check without contacting anything, so a plugin update
+stays visible without a lookup on every command. The
+`mvt plugins check-updates` command checks immediately, regardless of when the
+last check happened.
+
+The automatic check is skipped when the `--disable-update-check` option is
+used, when `NETWORK_ACCESS_ALLOWED` is disabled in the MVT configuration, and
+when no plugins are installed.
+
+## Plugins Installed From a Repository
+
+A plugin installed with `pip install "mvt-plugin-example @ git+<url>"` is
+checked by asking the remote repository which commit the installed revision
+points at now. MVT runs git and ssh in batch mode, so a repository which needs
+credentials MVT does not already have fails the check instead of prompting for
+them. The check is skipped silently when git is not available, when the
+repository cannot be reached, and when access to it is denied.
+
+How the plugin was installed decides what an update means:
+
+- A plugin installed from a branch is reported as outdated when the branch has
+  moved past the installed commit.
+- A plugin installed from a specific commit or a tag is pinned. It is never
+  reported as outdated, however far the branch it came from moves on.
+
+Pinning a plugin to a commit or a tag is therefore the way to keep the modules
+used across an investigation stable.