fix: ensure cwd for conversation and sandbox are separate concerns (#3874)

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


71038381aa/codex-rs/core/src/seatbelt.rs (L19-L25)


71038381aa/codex-rs/core/src/landlock.rs (L16-L23)

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.

codex-rs/core/src/shell.rs

@@ -349,6 +349,7 @@ mod tests {
                 },
                 SandboxType::None,
                 &SandboxPolicy::DangerFullAccess,
+                temp_home.path(),
                 &None,
                 None,
             )
@@ -455,6 +456,7 @@ mod macos_tests {
                 },
                 SandboxType::None,
                 &SandboxPolicy::DangerFullAccess,
+                temp_home.path(),
                 &None,
                 None,
             )