tui: remove the voice-input crate feature (#16467)

## Why

`voice-input` is the only remaining TUI crate feature, but it is also a
default feature and nothing in the workspace selects it explicitly. In
practice it is just acting as a proxy for platform support, which is
better expressed with target-specific dependencies and cfgs.

## What changed

- remove the `voice-input` feature from `codex-tui`
- make `cpal` a normal non-Linux target dependency
- replace the feature-based voice and audio cfgs with pure
Linux-vs-non-Linux cfgs
- shrink the workspace-manifest verifier allowlist to remove the
remaining `codex-tui` exception

## How tested

- `python3 .github/scripts/verify_cargo_workspace_manifests.py`
- `cargo test -p codex-tui`
- `just bazel-lock-check`
- `just argument-comment-lint -p codex-tui`

codex-rs/tui/src/app_command.rs

@@ -121,10 +121,7 @@ impl AppCommand {
         Self(Op::RealtimeConversationStart(params))
     }
 
-    #[cfg_attr(
-        any(target_os = "linux", not(feature = "voice-input")),
-        allow(dead_code)
-    )]
+    #[cfg_attr(target_os = "linux", allow(dead_code))]
     pub(crate) fn realtime_conversation_audio(params: ConversationAudioParams) -> Self {
         Self(Op::RealtimeConversationAudio(params))
     }