Guided /mcp command: add and verify MCP servers from the CLI #403

Summary

With the config writer (#402) in place, this issue adds the user-facing half of the MCP install helper (#310): a guided /mcp command in the CLI's standalone mode that takes a user from "AURA is installed" to "AURA can successfully use tools against my data" without hand-authoring TOML.

Scope

• /mcp — list the configured MCP servers for the active config with their transport and, when available, live connection status.
• /mcp add — a guided, deterministic wizard (no LLM in the loop for credentials):
  • Pick from a small curated catalog of blessed servers (each with prerequisites shown up front and the access AURA will require) or define a custom server (transport, url/cmd, headers).
  • Credentials are collected with masked input and stored as {{ env.VAR }} placeholders in the TOML; the secret value lands in the project .env, never in config.toml.
  • Config is updated in place via aura_config::writer::upsert_mcp_server, preserving unrelated settings.
  • An automatic connection and tool-discovery check runs against the newly added server and reports connected/failed status plus tool count.
  • On success, print a starter prompt that exercises the new integration.

Non-goals

• HTTP mode (--api-url): the config lives server-side; a mutation endpoint is a separate discussion.
• Editing or removing existing servers (/mcp remove etc.).
• Changing worker mcp_filter semantics ([FEATURE]: Empty worker mcp_filter grants access to all MCP tools #378).