chore(arg0): advisory-lock janitor for codex tmp paths (#10039)

## Description

### What changed
- Switch the arg0 helper root from `~/.codex/tmp/path` to
`~/.codex/tmp/path2`
- Add `Arg0PathEntryGuard` to keep both the `TempDir` and an exclusive
`.lock` file alive for the process lifetime
- Add a startup janitor that scans `path2` and deletes only directories
whose lock can be acquired

### Tests
- `cargo clippy -p codex-arg0`
- `cargo clippy -p codex-core`
- `cargo test -p codex-arg0`
- `cargo test -p codex-core`

codex-rs/core/tests/suite/mod.rs

@@ -1,16 +1,57 @@
 // Aggregates all former standalone integration tests as modules.
+use std::ffi::OsString;
+
+use codex_arg0::Arg0PathEntryGuard;
 use codex_arg0::arg0_dispatch;
 use ctor::ctor;
 use tempfile::TempDir;
 
+struct TestCodexAliasesGuard {
+    _codex_home: TempDir,
+    _arg0: Arg0PathEntryGuard,
+    _previous_codex_home: Option<OsString>,
+}
+
+const CODEX_HOME_ENV_VAR: &str = "CODEX_HOME";
+
 // This code runs before any other tests are run.
 // It allows the test binary to behave like codex and dispatch to apply_patch and codex-linux-sandbox
 // based on the arg0.
 // NOTE: this doesn't work on ARM
 #[ctor]
-pub static CODEX_ALIASES_TEMP_DIR: TempDir = unsafe {
+pub static CODEX_ALIASES_TEMP_DIR: TestCodexAliasesGuard = unsafe {
     #[allow(clippy::unwrap_used)]
-    arg0_dispatch().unwrap()
+    let codex_home = tempfile::Builder::new()
+        .prefix("codex-core-tests")
+        .tempdir()
+        .unwrap();
+    let previous_codex_home = std::env::var_os(CODEX_HOME_ENV_VAR);
+    // arg0_dispatch() creates helper links under CODEX_HOME/tmp. Point it at a
+    // test-owned temp dir so startup never mutates the developer's real ~/.codex.
+    //
+    // Safety: #[ctor] runs before tests start, so no test threads exist yet.
+    unsafe {
+        std::env::set_var(CODEX_HOME_ENV_VAR, codex_home.path());
+    }
+
+    #[allow(clippy::unwrap_used)]
+    let arg0 = arg0_dispatch().unwrap();
+    // Restore the process environment immediately so later tests observe the
+    // same CODEX_HOME state they started with.
+    match previous_codex_home.as_ref() {
+        Some(value) => unsafe {
+            std::env::set_var(CODEX_HOME_ENV_VAR, value);
+        },
+        None => unsafe {
+            std::env::remove_var(CODEX_HOME_ENV_VAR);
+        },
+    }
+
+    TestCodexAliasesGuard {
+        _codex_home: codex_home,
+        _arg0: arg0,
+        _previous_codex_home: previous_codex_home,
+    }
 };
 
 #[cfg(not(target_os = "windows"))]