mind: use tokio-scoped for Mind/UI loop lifetimes

Both event loops borrow &mind through a scoped spawn — no Arc on
Mind needed. Interior Arcs on agent/shared stay (background spawns
need 'static), but event_loop::run() now takes &Arc refs instead
of cloned Arcs.

Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>

Cargo.lock

@@ -2953,6 +2953,7 @@ dependencies = [
  "tiktoken-rs",
  "tokio",
  "tokio-rustls",
+ "tokio-scoped",
  "tokio-util",
  "toml",
  "tracing",
@@ -4371,6 +4372,16 @@ dependencies = [
  "tokio",
 ]
 
+[[package]]
+name = "tokio-scoped"
+version = "0.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e4beb8ba13bc53ac53ce1d52b42f02e5d8060f0f42138862869beb769722b256"
+dependencies = [
+ "tokio",
+ "tokio-stream",
+]
+
 [[package]]
 name = "tokio-stream"
 version = "0.1.18"