Add plugin installation script and documentation

The new `install-plugin` script automates plugin installation to Obsidian vaults
with support for both vault paths and direct plugin directory targets. It
includes options for skipping builds and dry runs.

Documentation in IMPLEMENTATION.md now includes usage examples for the new
installation method alongside the existing manual installation instructions.

IMPLEMENTATION.md

@@ -265,6 +265,12 @@ Run `npm run build`, then copy the plugin folder containing `manifest.json` and
 - Linux: ~/.config/obsidian/plugins/
 - macOS: ~/Library/Application Support/obsidian/plugins/
 - Windows: %APPDATA%\obsidian\plugins\
+
+# Or install directly into a vault
+npm run install-plugin -- --vault /path/to/vault
+
+# Exact target directory also works
+npm run install-plugin -- --plugin-dir /path/to/vault/.obsidian/plugins/obsidian-remarkable
 ```
 
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