Add packaged plugin support (.swiftbar directories)

[melonamin]

Summary

Closes #387

Adds support for packaged plugins — .swiftbar directories that bundle a main script with supporting files (libraries, assets, resources).

• New PackagedPlugin type — full Plugin protocol conformance with thread-safe metadata, timer lifecycle, and plugin variable support
• Directory traversal — PluginManager detects .swiftbar directories as plugin bundles and skips descending into their contents
• Working directory — packaged plugins execute with their package directory as cwd, so scripts can use relative paths (e.g., source ./lib/utils.sh)
• Environment variable — SWIFTBAR_PLUGIN_PACKAGE_PATH is set to the package directory path
• UTI registration — .swiftbar directories are registered as com.ameba.SwiftBar.PluginPackage so Finder treats them as opaque bundles

Plugin structure

my-plugin.swiftbar/
├── plugin.5m.sh      # Entry point (plugin.*)
├── lib/              # Helper scripts/libraries
└── assets/           # Data files, images, etc.

Ported from

This is a clean manual port of the packaged_plugins branch onto current main, preserving all recent features (menu diffing, folding menus, plugin variables, stdin support) that would have been lost in a rebase.

Test plan

• Place a .swiftbar directory with a plugin.sh entry point in the plugin folder — verify it loads and displays in the menu bar
• Verify source ./lib/helper.sh works inside a packaged plugin (confirms working directory is set correctly)
• Verify SWIFTBAR_PLUGIN_PACKAGE_PATH is set in the plugin's environment
• Verify regular (non-packaged) plugins still work as before
• Verify .swiftbar directories appear as bundles in Finder
• Verify refresh interval works via entry point naming (e.g., plugin.30s.sh)