feat(companion): support remote pack installation and multiple asset packs

[mhenke]

Originally tracked in issue #561 by @alvinunreal

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Problem

Currently, the Desktop Companion app ships with a single hardcoded asset pack (character design, animations, and spritesheets) bundled directly into the compiled binary via include_bytes!. Adding more character themes, animations, or spritesheet packs directly to the source code would significantly bloat the companion executable/plugin bundle, making downloads slower and distribution less efficient.

Goal

Implement support for installable packs—specifically remote/downloadable asset packs—so users can easily add, switch, and download new character design, animation, and spritesheet packs without bloating the core companion package/binary.

Suggested Scope

1. Pack Manifest & Metadata:
   • Define a standard format (e.g., JSON/YAML) to describe an asset pack, detailing the pack name, author, version, target/agent mappings, and the assets themselves.
2. Remote Install Command/Workflow:
   • Introduce a command-line interface or configuration option to install packs from a remote URL or repository (e.g., oh-my-opencode-slim companion install-pack <url-or-name>).
3. Local Cache & Storage:
   • Determine a standard, persistent storage directory under user data/cache directories (e.g., ~/.local/share/opencode/storage/oh-my-opencode-slim/packs/) to store downloaded packs.
4. Versioning & Compatibility:
   • Ensure the companion and plugin runtimes can validate that a given asset pack is compatible with the current version of the companion protocol.
5. Validation & Checksums:
   • Use cryptographic hashes (e.g., SHA256 checksums) to verify remote asset pack integrity upon download.
6. Documentation:
   • Provide clear instructions on how users can install packs, and documentation for developers on how to create, format, and host custom companion asset packs.
7. Default Bundled Pack Behavior:
   • Keep a basic default pack bundled or pre-installed so the companion works out-of-the-box without requiring initial internet downloads.

Acceptance Criteria

• Users can list, install, update, and switch between multiple asset packs via CLI or config file.
• Remote asset packs can be downloaded dynamically and are stored in a designated local cache/storage folder.
• Installed packs are validated against a schema/manifest and checksum verified.
• The companion app loads assets dynamically at runtime from the active pack's local directory instead of relying solely on hardcoded include_bytes!.
• If no external pack is installed or selected, the companion falls back gracefully to a built-in default asset pack.
• Proper error handling is implemented for failed downloads, corrupt archives, or incompatible pack versions.