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/exec/src/cli_tests.rs

@@ -70,3 +70,13 @@ fn parses_config_isolation_flags() {
     assert!(cli.ignore_user_config);
     assert!(cli.ignore_rules);
 }
+
+#[test]
+fn removed_full_auto_flag_reports_migration_path() {
+    let cli = Cli::parse_from(["codex-exec", "--full-auto", "summarize"]);
+
+    assert_eq!(
+        cli.removed_full_auto_warning(),
+        Some("warning: `--full-auto` is deprecated; use `--sandbox workspace-write` instead.")
+    );
+}