fix: ensure cwd for conversation and sandbox are separate concerns

[bolinfest]

Previous to this PR, both of these functions take a single cwd:

codex/codex-rs/core/src/seatbelt.rs

Lines 19 to 25 in 7103838

	pub async fn spawn_command_under_seatbelt(
	command: Vec<String>,
	sandbox_policy: &SandboxPolicy,
	cwd: PathBuf,
	stdio_policy: StdioPolicy,
	mut env: HashMap<String, String>,
	) -> std::io::Result<Child> {

codex/codex-rs/core/src/landlock.rs

Lines 16 to 23 in 7103838

	pub async fn spawn_command_under_linux_sandbox<P>(
	codex_linux_sandbox_exe: P,
	command: Vec<String>,
	sandbox_policy: &SandboxPolicy,
	cwd: PathBuf,
	stdio_policy: StdioPolicy,
	env: HashMap<String, String>,
	) -> std::io::Result<Child>

whereas cwd and sandbox_cwd should be set independently (fixed in this PR).

Added sandbox_distinguishes_command_and_policy_cwds() to codex-rs/exec/tests/suite/sandbox.rs to verify this.

[fouad-openai]

would recommend sandbox_cwd → workspace_root but non-blocking