feat(tools): add external tool registration API

[Yyu-ang]

This is the external-tool API split requested during the review of #92.

What this PR adds

A public API that allows other Zotero plugins to register custom MCP tools through:

Zotero.ZoteroMCP.api

The API exposes:

• registerTool(def): boolean
• unregisterTool(name): boolean
• unregisterAllTools(pluginID): number
• getRegisteredTools()
• isToolRegistered(name)
• onToolListChanged(callback)

Tool registration

External tool definitions support:

• tool name
• description
• JSON input schema
• async or synchronous handler
• optional pluginID
• optional enabled state

Registered tools are exposed through the existing MCP tools/list and dispatched through the existing MCP server.

Safety and compatibility

• external tool names must match the registry naming rules
• duplicate registrations are rejected
• built-in MCP tool names cannot be overridden
• export_bibliography, get_citation, and list_citation_styles are reserved
• the planned sync_bib and cite names are also reserved
• pluginID remains optional
• no separate enable/disable preference or registration-handle API is introduced

This keeps the public API compatible with the implementation discussed in #92.

Lifecycle

The registry is process-wide and survives MCP server restarts. Third-party plugins can unregister individual tools or clean up all tools associated with their pluginID during shutdown.

The PR also includes:

• TypeScript API typings
• an external-plugin example
• English and Chinese documentation
• registry regression tests

Scope

This PR contains only the external tool registration API.

It does not include:

• citation/export implementation
• sync_bib / cite
• semantic search dialog
• version or release metadata

Validation

• rebased onto the current main
• full npm test passes
• production npm run build passes
• registry tests cover registration, removal, optional pluginID, duplicate/reserved names, and sync_bib / cite reservation

Related: #92