Feature: package/extension system for sharing community-developed features (like Pi packages)

Feature request
Add a package/extension system so community-developed features can be easily shared and installed, analogous to the package system in the Pi coding agent.

What it should do

• A first-class way to package, publish, and install extensions (skills, tools, providers, commands, UI additions) without a full jcode fork/build.
• Simple install/publish UX (e.g. an index/registry plus a CLI command such as jcode pkg install <name>), and a clear manifest format describing an extension (name, version, dependencies, entrypoints, supported jcode versions).
• Versioning and an explicit trust/review story, since extensions execute with user privileges.

Why it benefits all users

• The issue tracker is already receiving requests where users are re-implementing the same provider integrations (e.g. commandcode #741) and features because there is no supported distribution path.
• A package system lets one person build a provider or tool once and share it, instead of many users self-developing overlapping functionality (and shipping their own fork builds, which the maintainer then has to vet).
• Centralizes where community contributions live and makes them installable/reviewable rather than scattered as uncommitted fork patches.

Suggested scope

• Extension manifest + discovery.
• jcode pkg subcommand (install/uninstall/list/publish/update).
• A trust/review mechanism before installing an extension.
• Docs on authoring extensions.

This complements the existing skill system (which is local) and would avoid duplicating useful features across users.

Start by tracing the existing local skill system and CLI command structure, then determine where a manifest, discovery, and the jcode pkg subcommands would fit. Done means extensions can be described, published, installed, updated, and reviewed with documented trust expectations, without requiring a fork or rebuild.