Refuse grant paths that cannot be mounted

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

crates/brokerd/src/grants.rs

@@ -260,6 +260,14 @@ fn check_grant(grant: &LoadedGrant, problems: &mut Vec<GrantProblem>) {
                 None,
                 format!("{:?} is not a valid absolute path", path),
             );
+        } else if path.contains([':', ',']) {
+            // Mounted as `--volume=<path>:<path>:ro`, where both are separators.
+            push(
+                problems,
+                file.clone(),
+                None,
+                format!("{:?} cannot be mounted: it contains ':' or ','", path),
+            );
         }
     }
     // Rule 8: every host must be a valid host pattern.

crates/brokerd/tests/grants_mount.rs

@@ -0,0 +1,45 @@
+//! A grant path is mounted into the tool container as `--volume=<path>:<path>:ro`, so a path with
+//! `:` or `,` in it cannot be granted: the set is invalid, as for any other bad grant (M3b spec,
+//! section 3). Do not edit.
+
+#[path = "support/tmp.rs"]
+mod tmp;
+
+use brokerd::grants::load;
+use tmp::TempDir;
+
+fn grant_with_path(path: &str) -> String {
+    format!(
+        "tool = \"read_file\"\nmode = \"auto\"\nmax_taint = \"secret\"\n[constraints]\npaths = [{path:?}]\n"
+    )
+}
+
+#[test]
+fn a_path_with_a_colon_or_a_comma_makes_the_set_invalid() {
+    for path in ["/home/kyle/a:b", "/home/kyle/a,b", "/x:/y", "/n,ro"] {
+        let dir = TempDir::new("mount-bad");
+        dir.write("notes.toml", &grant_with_path(path));
+        let problems = load(dir.path()).expect_err(path);
+        assert_eq!(problems.len(), 1, "{path}: {problems:?}");
+        assert_eq!(problems[0].file, "notes.toml");
+        assert!(
+            problems[0].problem.contains("cannot be mounted"),
+            "{path}: {}",
+            problems[0].problem
+        );
+    }
+}
+
+#[test]
+fn other_punctuation_is_still_fine() {
+    for path in [
+        "/home/kyle/a b",
+        "/home/kyle/a;b",
+        "/home/kyle/a=b",
+        "/home/kyle/a.b-c_d",
+    ] {
+        let dir = TempDir::new("mount-ok");
+        dir.write("notes.toml", &grant_with_path(path));
+        assert!(load(dir.path()).is_ok(), "{path}");
+    }
+}