loopd, bxctl, inferproxy: read args_os instead of panicking

M3a review finding 7, for the other three roles. loopd keeps its config path
as a path; bxctl and inferproxy take text arguments and answer one that is
not UTF-8 with their usage.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/loopd/tests/args_os.rs

@@ -0,0 +1,36 @@
+//! A config path that is not UTF-8 is read as a path, not a panic (M3a review finding 7).
+
+use std::ffi::OsStr;
+use std::os::unix::ffi::OsStrExt;
+use std::process::Command;
+
+#[test]
+fn a_config_path_that_is_not_utf8_is_read_as_a_path() {
+    let path = std::env::temp_dir().join(OsStr::from_bytes(b"loopd-missing-\xff.toml"));
+    let out = Command::new(env!("CARGO_BIN_EXE_loopd"))
+        .args([
+            OsStr::new("selftest"),
+            OsStr::new("--config"),
+            path.as_os_str(),
+        ])
+        .output()
+        .unwrap();
+    assert_eq!(
+        out.status.code(),
+        Some(1),
+        "a config error, not a panic (101)"
+    );
+}
+
+#[test]
+fn a_flag_that_is_not_utf8_is_a_usage_error() {
+    let out = Command::new(env!("CARGO_BIN_EXE_loopd"))
+        .args([
+            OsStr::from_bytes(b"serve\xff"),
+            OsStr::new("--config"),
+            OsStr::new("x"),
+        ])
+        .output()
+        .unwrap();
+    assert_eq!(out.status.code(), Some(2));
+}