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>

docs/usage.md

@@ -32,8 +32,10 @@ jobs:
           #   --max-turns 10
           #   --model claude-4-0-sonnet-20250805
 
+          # Optional: add custom plugin marketplaces
+          # plugin_marketplaces: "https://github.com/user/marketplace1.git\nhttps://github.com/user/marketplace2.git"
           # Optional: install Claude Code plugins
-          # plugins: "plugin1,plugin2,plugin3"
+          # plugins: "code-review@claude-code-plugins\nfeature-dev@claude-code-plugins"
 
           # Optional: add custom trigger phrase (default: @claude)
           # trigger_phrase: "/claude"
@@ -76,7 +78,8 @@ jobs:
 | `allowed_non_write_users`        | **⚠️ RISKY**: Comma-separated list of usernames to allow without write permissions, or '\*' for all users. Only works with `github_token` input. See [Security](./security.md)         | No       | ""            |
 | `path_to_claude_code_executable` | Optional path to a custom Claude Code executable. Skips automatic installation. Useful for Nix, custom containers, or specialized environments                                         | No       | ""            |
 | `path_to_bun_executable`         | Optional path to a custom Bun executable. Skips automatic Bun installation. Useful for Nix, custom containers, or specialized environments                                             | No       | ""            |
-| `plugins`                        | Comma-separated list of Claude Code plugin names to install (e.g., `plugin1,plugin2,plugin3`). Plugins are installed before Claude Code execution                                      | No       | ""            |
+| `plugin_marketplaces`            | Newline-separated list of Claude Code plugin marketplace Git URLs to install from (e.g., see example in workflow above). Marketplaces are added before plugin installation             | No       | ""            |
+| `plugins`                        | Newline-separated list of Claude Code plugin names to install (e.g., see example in workflow above). Plugins are installed before Claude Code execution                                | No       | ""            |
 
 ### Deprecated Inputs