chore(cli) deprecate --full-auto (#20133)

## Summary
Starts the process of getting rid of `--full-auto`, with some
concessions:
1. Fully removes the command from the tui, since it just resolves to the
default permissions there, and encourages users to use the one-time
trust flow if they're not in a trusted repo.
2. Marks the command as deprecated in `codex exec`, in case users are
actively relying on this. We'll remove in an upcoming n+X release.
3. Cleans up some of the `codex sandbox` cli logic, to keep supporting
legacy sandbox policies for now.

This isn't the cleanest setup, but I think it is worthwhile to warn
users for one release before hard-removing it.

## Testing 
- [x] Updated unit tests

codex-rs/tui/src/lib.rs

@@ -697,12 +697,7 @@ pub async fn run_main(
         .cwd
         .clone()
         .filter(|_| matches!(app_server_target, AppServerTarget::Remote { .. }));
-    let (sandbox_mode, approval_policy) = if cli.full_auto {
-        (
-            Some(SandboxMode::WorkspaceWrite),
-            Some(AskForApproval::OnRequest),
-        )
-    } else if cli.dangerously_bypass_approvals_and_sandbox {
+    let (sandbox_mode, approval_policy) = if cli.dangerously_bypass_approvals_and_sandbox {
         (
             Some(SandboxMode::DangerFullAccess),
             Some(AskForApproval::Never),