[Feature] Standalone local MCP server so Claude Desktop / Cursor / any MCP client can use a local library #540

Problem

In local mode the agent tools are only reachable in-process, through the framework adapters
(as_openai_tools(), as_claude_mcp(), as_anthropic_tools()). Any MCP-speaking client outside
that Python process — Claude Desktop, Cursor, Cline, etc. — has no endpoint to connect to.
Cloud users can already attach such clients to the hosted MCP server (https://api.pageindex.ai/mcp);
local users cannot use their own library there at all. This is the open half of #137.

The README Today marks "MCP server" as Cloud-only (— in the Local column), which is accurate.

What already exists

• pageindex/agent_tools.py carries the cloud-identical tool contract plus full local
  implementations: _tool_specs() returns (name, description, schema, invoke) and
  tool_names() gives browse_documents, get_document, get_document_structure,
  get_page_content (+ remove_document behind include_management).
• integrations/openai_agents.py and integrations/claude_agent_sdk.py already wrap those exact
  specs in an in-process MCP server (CallToolResult.model_validate({"content": blocks, "isError": is_error})), so the result conversion is proven — only a standalone process is missing.
• mcp>=1.19.0,<3 is already a required dependency, so no new dependency would be added.

Proposal (v1)

1. New pageindex/mcp_server.py: a server built over _tool_specs(...) for a
   PageIndexLocalClient, with handlers doing await asyncio.to_thread(invoke, arguments) and
   returning CallToolResult (server-side isError carried through).
2. stdio transport only.
3. A pageindex-mcp console entry point ([tool.poetry.scripts]), e.g.
   pageindex-mcp --storage-path <dir> [--management], config otherwise via the same env vars
   the SDK uses.
4. Tests: an in-process MCP session test (tools/list + one tool call round-trip) and a stdio
   smoke test.
5. README: flip the Local column's "MCP server" cell and add a short client-config snippet.

Client config the server would enable:

{
  "mcpServers": {
    "pageindex-local": {
      "command": "pageindex-mcp",
      "args": ["--storage-path", "/path/to/index-store"]
    }
  }
}