Added feature switch to disable animations in TUI (#6870)

This PR adds support for a new feature flag `tui.animations`. By
default, the TUI uses animations in its welcome screen, "working"
spinners, and "shimmer" effects. This animations can interfere with
screen readers, so it's good to provide a way to disable them.

This change is inspired by [a
PR](https://github.com/openai/codex/pull/4014) contributed by @Orinks.
That PR has faltered a bit, but I think the core idea is sound. This
version incorporates feedback from @aibrahim-oai. In particular:
1. It uses a feature flag (`tui.animations`) rather than the unqualified
CLI key `no-animations`. Feature flags are the preferred way to expose
boolean switches. They are also exposed via CLI command switches.
2. It includes more complete documentation.
3. It disables a few animations that the other PR omitted.

codex-rs/tui/src/exec_cell/model.rs

@@ -28,11 +28,15 @@ pub(crate) struct ExecCall {
 #[derive(Debug)]
 pub(crate) struct ExecCell {
     pub(crate) calls: Vec<ExecCall>,
+    animations_enabled: bool,
 }
 
 impl ExecCell {
-    pub(crate) fn new(call: ExecCall) -> Self {
-        Self { calls: vec![call] }
+    pub(crate) fn new(call: ExecCall, animations_enabled: bool) -> Self {
+        Self {
+            calls: vec![call],
+            animations_enabled,
+        }
     }
 
     pub(crate) fn with_added_call(
@@ -56,6 +60,7 @@ impl ExecCell {
         if self.is_exploring_cell() && Self::is_exploring_call(&call) {
             Some(Self {
                 calls: [self.calls.clone(), vec![call]].concat(),
+                animations_enabled: self.animations_enabled,
             })
         } else {
             None
@@ -112,6 +117,10 @@ impl ExecCell {
             .and_then(|c| c.start_time)
     }
 
+    pub(crate) fn animations_enabled(&self) -> bool {
+        self.animations_enabled
+    }
+
     pub(crate) fn iter_calls(&self) -> impl Iterator<Item = &ExecCall> {
         self.calls.iter()
     }