Add external MCP server support (stdio JSON-RPC)

Lets the AI connect to any external stdio MCP server (e.g. safaridriver
--mcp) configured in Settings, with tools auto-discovered and prefixed
by server slug. Includes crash detection with backoff restart (5s/15s/30s)
and a Settings UI to add/enable/disable/remove servers.

Fixes the temp-dir allowlist in MCPService.isPathAllowed to also match
/tmp and /private/tmp (not just NSTemporaryDirectory(), which resolves
to a different per-user Darwin temp dir) so the MCP file tools can
actually read files external servers and image generation write there.
Also switches the Add Server sheet's argument parsing to a quote-aware
tokenizer so args containing spaces survive intact.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

oAI/Utilities/Logging.swift

@@ -153,4 +153,5 @@ enum Log {
     nonisolated static let search   = AppLogger(subsystem: subsystem, category: "search")
     nonisolated static let ui       = AppLogger(subsystem: subsystem, category: "ui")
     nonisolated static let general  = AppLogger(subsystem: subsystem, category: "general")
+    nonisolated static let extMcp   = AppLogger(subsystem: subsystem, category: "ext-mcp")
 }