codeSplitters: show handle invites to the invitee in the app, and check pi's MCP config line #539

Follow-up to #526.

• A handle invite is listed by GET /api/invites (pending invites to your handle, with links), but the page doesn't show it yet: the invitee only learns of it if the owner sends the link. Show them on the invitee's profile or in the top bar.
• The agent door's MCP line for pi is the generic {"mcpServers": {"codesplitters": {url, headers}}} shape (labelled "pi, or any MCP client"). Check it against pi's real MCP config and adjust mcpLines in apps/codesplitters/src/invites.ts.