feat: change plugins input from comma-separated to newline-separated (#644)

* feat: change plugins input from comma-separated to newline-separated

Changes:
- Update parsePlugins() to split by newline instead of comma for consistency with marketplaces input
- Update action.yml and base-action/action.yml with newline-separated format and realistic plugin examples
- Add plugin_marketplaces documentation to docs/usage.md
- Update all unit tests to match new installPlugins() signature (marketplaces, plugins, executable)
- Improve JSDoc comments for parsePlugins() and installPlugin() functions
- All 25 install-plugins tests passing

Breaking change: Users must update their workflows to use newline-separated format:
  Before: plugins: "plugin1,plugin2"
  After: plugins: "plugin1\nplugin2"

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

* test: add comprehensive marketplace functionality tests

Critical fix: All previous tests passed undefined as marketplacesInput parameter,
leaving the entire marketplace functionality completely untested.

Added 13 new tests covering:
- Single marketplace installation
- Multiple marketplaces with newline separation
- Marketplace + plugin installation order verification
- Marketplace URL validation (format, protocol, .git extension)
- Whitespace and empty entry handling
- Error handling for marketplace operations
- Custom executable path for marketplace operations

Test coverage: 38 tests (was 25), 81 expect calls (was 50)
All tests passing ✅

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

---------

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

base-action/action.yml

@@ -56,7 +56,7 @@ inputs:
     required: false
     default: ""
   plugins:
-    description: "Comma-separated list of Claude Code plugin names to install (e.g., 'plugin1,plugin2,plugin3')"
+    description: "Newline-separated list of Claude Code plugin names to install (e.g., 'code-review@claude-code-plugins\nfeature-dev@claude-code-plugins')"
     required: false
     default: ""
   plugin_marketplaces: