refactor(plugins): split PluginStoreManager into mixins (#659)

* refactor(plugins): split PluginStoreManager into mixins

src/plugin_system/store_manager.py (2,977 lines) keeps the class, its
shared state, locks, the uninstall registry, directory lookup and
uninstall; its methods are split by area into:
- store_registry.py (_RegistryMixin): registry, GitHub metadata, search,
  manifest validation
- store_install.py (_InstallMixin): install paths and dependencies
- store_update.py (_UpdateMixin): updates, rollback, local git state

Pure move: all 56 members are byte-identical (checked with ast) and the
assembled class has exactly the same attributes as before (checked at
runtime). PluginStoreManager is imported from store_manager.py as before.
Tests that patched shared modules (subprocess, requests, tempfile, shutil)
through store_manager now reach them through the module whose code they
exercise; a source-text contract test reads all store_*.py modules.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* chore: annotate findings the split moved into new store modules

subprocess imports and a list-form git clone (no shell), and the config
template's placeholder token string -- existing code that Codacy reported
as new because it moved. Annotated with the repo's nosec/nosemgrep style.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* chore: annotate the default-branch git clone the split moved

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

docs/PLUGIN_DEPENDENCY_GUIDE.md

@@ -25,7 +25,7 @@ which runs as root.** Anything installed only into another user's
 The web interface is not root, so it installs through a narrow sudo helper:
 
 1. `PluginStoreManager._install_dependencies()`
-   (`src/plugin_system/store_manager.py`) calls
+   (`src/plugin_system/store_install.py`) calls
    `install_requirements_file()` (`src/common/permission_utils.py`).
 2. That runs `sudo -n bash scripts/fix_perms/safe_pip_install.sh <plugin>/requirements.txt`.
    The helper checks the path is the project's own `requirements.txt` or a
@@ -154,7 +154,7 @@ For more, see the [Plugin Dependency Troubleshooting Guide](PLUGIN_DEPENDENCY_TR
 ## Files to Reference
 
 - Service units: `systemd/ledmatrix.service`, `systemd/ledmatrix-web.service`
-- Store installs: `src/plugin_system/store_manager.py` (`_install_dependencies`)
+- Store installs: `src/plugin_system/store_install.py` (`_install_dependencies`)
 - Root install helper: `src/common/permission_utils.py` (`install_requirements_file`), `scripts/fix_perms/safe_pip_install.sh`
 - Load-time installs: `src/plugin_system/plugin_loader.py` (`install_dependencies`)
 - Sudo rules: `scripts/install/lib_sudoers.sh` (written by `first_time_install.sh`