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.

docs/example-config.md

@@ -140,13 +140,16 @@ file_opener = "vscode"
 # Examples: false | ["agent-turn-complete", "approval-requested"]
 notifications = false
 
-# Suppress internal reasoning events from output (default: false)
+# Enables welcome/status/spinner animations. Default: true
+animations = true
+
+# Suppress internal reasoning events from output. Default: false
 hide_agent_reasoning = false
 
-# Show raw reasoning content when available (default: false)
+# Show raw reasoning content when available. Default: false
 show_raw_agent_reasoning = false
 
-# Disable burst-paste detection in the TUI (default: false)
+# Disable burst-paste detection in the TUI. Default: false
 disable_paste_burst = false
 
 # Track Windows onboarding acknowledgement (Windows only). Default: false