docs: clarify plugin paths and fix systemd manual install bug

PLUGIN_DEVELOPMENT_GUIDE.md
- Added a "Plugin directory note" callout near the top explaining
  the plugins/ vs plugin-repos/ split:
  - Dev workflow uses plugins/ (where dev_plugin_setup.sh creates
    symlinks)
  - Production / Plugin Store uses plugin-repos/ (the configurable
    default per config.template.json:130)
  - The plugin loader falls back to plugins/ so dev symlinks are
    picked up automatically (schema_manager.py:77)
  - User can set plugins_directory to "plugins" in the General tab
    if they want both to share a directory

CLAUDE.md
- The Project Structure section had plugins/ and plugin-repos/
  exactly reversed:
  - Old: "plugins/ - Installed plugins directory (gitignored)"
         "plugin-repos/ - Development symlinks to monorepo plugin dirs"
  - Real: plugin-repos/ is the canonical Plugin Store install
    location and is not gitignored. plugins/* IS gitignored
    (verified in .gitignore) and is the legacy/dev location used by
    scripts/dev/dev_plugin_setup.sh.
  Reversed the descriptions and added line refs.

systemd/README.md
- "Manual Installation" section told users to copy the unit file
  directly to /etc/systemd/system/. Verified the unit file in
  systemd/ledmatrix.service contains __PROJECT_ROOT_DIR__
  placeholders that the install scripts substitute at install time.
  A user following the manual steps would get a service that fails
  to start with "WorkingDirectory=__PROJECT_ROOT_DIR__" errors.
  Added a clear warning and a sed snippet that substitutes the
  placeholder before installing.

src/common/README.md
- Was missing 2 of the 11 utility modules in the directory
  (verified with ls): permission_utils.py and cli.py. Added brief
  descriptions for both.

Out-of-scope code bug found while auditing (flagged but not fixed):
- scripts/dev/dev_plugin_setup.sh:9 sets PROJECT_ROOT="$SCRIPT_DIR"
  which resolves to scripts/dev/, not the project root. This means
  the script's PLUGINS_DIR resolves to scripts/dev/plugins/ instead
  of the project's plugins/ — confirmed by the existence of
  scripts/dev/plugins/of-the-day/ from prior runs. Real fix is to
  set PROJECT_ROOT="$(cd "$SCRIPT_DIR/../.." && pwd)". Not fixing in
  this docs PR.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/common/README.md

@@ -71,6 +71,17 @@ General-purpose utility functions:
 - Boolean parsing
 - Logger creation (deprecated - use `src.logging_config.get_logger()`)
 
+## Permission Utilities (`permission_utils.py`)
+
+Helpers for ensuring directory permissions and ownership are correct
+when running as a service (used by `CacheManager` to set up its
+persistent cache directory).
+
+## CLI Helpers (`cli.py`)
+
+Shared CLI argument parsing helpers used by `scripts/dev/*` and other
+command-line entry points.
+
 ## Best Practices
 
 1. **Use centralized logging**: Import from `src.logging_config` instead of creating loggers directly