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/application/src/plugin/mod.rs

@@ -2443,6 +2443,10 @@ fn workspace_exec_sandbox(
         abs_root: root.as_str().to_owned(),
         access: project_access,
     }];
+    allowed.push(domain::PathGrant {
+        abs_root: "/dev/null".to_owned(),
+        access: domain::PathAccess::RO | domain::PathAccess::RW,
+    });
     for system_root in ["/bin", "/usr", "/lib", "/lib64", "/etc"] {
         allowed.push(domain::PathGrant {
             abs_root: system_root.to_owned(),
@@ -6063,6 +6067,29 @@ mod tests {
         }
     }
 
+    #[test]
+    fn workspace_exec_read_sandbox_grants_dev_null_without_widening_project_access() {
+        let root = ProjectPath::new("/workspace/example").unwrap();
+        let sandbox = workspace_exec_sandbox(&root, PluginWorkspaceExecEffect::Read);
+
+        assert_eq!(
+            sandbox
+                .allowed
+                .iter()
+                .find(|grant| grant.abs_root == root.as_str())
+                .map(|grant| grant.access),
+            Some(domain::PathAccess::RO | domain::PathAccess::EXEC)
+        );
+        assert_eq!(
+            sandbox
+                .allowed
+                .iter()
+                .find(|grant| grant.abs_root == "/dev/null")
+                .map(|grant| grant.access),
+            Some(domain::PathAccess::RO | domain::PathAccess::RW)
+        );
+    }
+
     #[tokio::test]
     async fn workspace_exec_allows_matching_rule_clamps_limits_and_audits_metadata() {
         let (project_id, tool, runner, audit) = workspace_exec_fixture();