URL Handlers for Plugin Installation from Multiple Sources and Marketplaces

[digitarald]

VS Code currently supports vscode:// URI handlers for extension installation and vscode:mcp/install for MCP server installation. We should extend this pattern to provide comprehensive URL handlers for plugin installation that support:

1. Installing plugins from a GitHub repo — A URL handler that accepts a GitHub repository URL and installs the plugin directly
2. Installing plugins from any source — Support for arbitrary plugin sources beyond the default marketplace
3. Adding new marketplaces — A URL handler to register additional plugin marketplaces
4. Combo URL handler — A single URL that can both add a marketplace and install a specific plugin from it in one step

[connor4312]

Implemented in #302556. Here are example URIs using \�nthropics/claude-code\ as a marketplace and \hookify\ as a plugin:

Install a plugin from source (plain text)

\
vscode://chat-plugin/install?source=anthropics/claude-code
\\

Install a plugin from source (base64-encoded)

\
vscode://chat-plugin/install?source=YW50aHJvcGljcy9jbGF1ZGUtY29kZQ==
\
(\YW50aHJvcGljcy9jbGF1ZGUtY29kZQ==\ = base64 of \�nthropics/claude-code)

Add a marketplace (plain text)

\
vscode://chat-plugin/add-marketplace?ref=anthropics/claude-code
\\

Add a marketplace (base64-encoded)

\
vscode://chat-plugin/add-marketplace?ref=YW50aHJvcGljcy9jbGF1ZGUtY29kZQ==
\\

Both plain-text \owner/repo\ and base64-encoded values are supported. Base64 is tried first, with fallback to raw value if the decoded string isn't a valid marketplace reference.

Note: the \install\ path calls \installPluginFromSource, which clones the repo and discovers plugins via \marketplace.json. If there are multiple plugins (e.g. \hookify), the user gets a quick-pick to select which one. A direct deep-link to a specific plugin within a marketplace (e.g. ?source=anthropics/claude-code&plugin=hookify) is not yet supported but could be added.