feat: add plugin system for installing extensions from GitHub repos

[roomote-v0]

Related GitHub Issue

Closes: #11974

Description

This PR attempts to address Issue #11974 by introducing an MVP plugin system that acts as a distribution and installation mechanism on top of existing Roo Code extension points. Feedback and guidance are welcome.

Key implementation details:

• Plugin manifest schema (packages/types/src/plugin.ts): Zod-validated plugin.json format that defines what a plugin bundles (commands, modes, MCP servers, skills)
• GitHubSource (src/services/plugin/GitHubSource.ts): Fetches plugin manifests and files from GitHub repos via raw content API. Supports owner/repo and owner/repo@ref formats
• PluginInstaller (src/services/plugin/PluginInstaller.ts): Handles installing/removing each extension point type into the correct locations (.roo/commands/, .roomodes, mcp_settings.json, .roo/skills/)
• PluginManager (src/services/plugin/PluginManager.ts): Orchestrates install/remove/list with tracking via plugins.json
• Built-in /plugin command (src/services/command/built-in-commands.ts): Provides agent instructions for plugin management (install, remove, list)

Design decisions:

• Plugins are GitHub repos with a plugin.json manifest at the root
• Installation tracking uses plugins.json (project or global scope)
• Each extension point type is installed to its standard location
• No new dependencies added - uses existing fetch, yaml, zod

Test Procedure

• 25 new unit tests covering GitHubSource (18 tests) and PluginManager (7 tests)
• All existing command tests (27) continue to pass
• All lint checks pass across 14 packages
• All type checks pass across 14 packages

Run tests:

cd src && npx vitest run services/plugin/__tests__/GitHubSource.spec.ts
cd src && npx vitest run services/plugin/__tests__/PluginManager.spec.ts

Pre-Submission Checklist

• Issue Linked: This PR is linked to an approved GitHub Issue (see "Related GitHub Issue" above).
• Scope: My changes are focused on the linked issue (one major feature/fix per PR).
• Self-Review: I have performed a thorough self-review of my code.
• Testing: New and/or updated tests have been added to cover my changes (if applicable).
• Documentation Impact: I have considered if my changes require documentation updates (see "Documentation Updates" section below).
• Contribution Guidelines: I have read and agree to the Contributor Guidelines.

Documentation Updates

Plugin manifest format documentation and usage examples should be added as part of follow-up work once the design is reviewed and approved.

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