feat(mcp): --mcp-stdio launches the headless stdio adapter (T10.4, FR-MCP-6)

Checked before whispassist_lib::run() builds the Tauri app -- the flag
routes straight to run_mcp_stdio() (next commit) and returns instead of
opening a window.

src-tauri/src/main.rs

@@ -3,5 +3,14 @@
 #![cfg_attr(not(debug_assertions), windows_subsystem = "windows")]
 
 fn main() {
+    // `whispassist.exe --mcp-stdio` (FR-MCP-6): the stdio "adapter the agent
+    // spawns" is this same binary, in headless mode -- it serves one MCP
+    // session over its own stdin/stdout and exits, instead of opening the
+    // GUI window. A coding agent's MCP client config spawns this exact
+    // command line (see `mcp_status`/`set_mcp_enabled`'s returned endpoint).
+    if std::env::args().any(|a| a == "--mcp-stdio") {
+        whispassist_lib::run_mcp_stdio();
+        return;
+    }
     whispassist_lib::run();
 }