fix(application,infrastructure): grant /dev/null RO|RW in workspace_exec_sandbox so CLIs can open it O_RDWR — #354 (QA verte)

Cause: the workspace.execTool sandbox only granted the project root
plus read-only system roots, so CLI tools like git failed when opening
/dev/null with O_RDWR (redirect/exec 3<>/dev/null) inside sandboxed
runs.
Fix: workspace_exec_sandbox pushes one precise /dev/null PathGrant with
RO|RW access; project read effect stays RO|EXEC and no blanket /dev
grant is added, so the widening is limited to the null device;
GitPlugin untouched. Tests: application asserts the read sandbox keeps
project RO|EXEC while granting /dev/null RO|RW
(workspace_exec_read_sandbox_grants_dev_null_without_widening_project_access);
infrastructure proves the local runner executes exec 3<>/dev/null
(local_runner_allows_cli_to_open_dev_null_for_read_and_write).
QA verte reconfirmée par Git: cargo test -p application workspace_exec
5/5, -p infrastructure local_runner_allows_cli_to_open_dev_null 1/1,
-p infrastructure workspace_tool::tests 5/5, cargo fmt --all --check,
git diff --check.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/infrastructure/src/workspace_tool.rs

@@ -263,4 +263,18 @@ mod tests {
         assert!(result.stdout_truncated);
         assert!(!result.stderr_truncated);
     }
+
+    #[cfg(unix)]
+    #[tokio::test]
+    async fn local_runner_allows_cli_to_open_dev_null_for_read_and_write() {
+        let runner = LocalWorkspaceToolRunner::new(Arc::new(crate::sandbox::NoopSandbox));
+        let result = runner
+            .run(spec(&["-c", "exec 3<>/dev/null; printf ok"], 1_000, 32))
+            .await
+            .unwrap();
+
+        assert_eq!(result.exit_code, Some(0));
+        assert_eq!(result.stdout, b"ok");
+        assert!(result.stderr.is_empty());
+    }
 }