Specify and plan M3b: the runner and the tools

A draft spec for the owner's review and 13 offline tasks with their given
tests: shared tool arguments and host rules in proto, the sealed fetch
target (M3a finding 14), the toolkit tools and SOCKS5 egress proxy, and
brokerd's [runner], podman argument lists, runtime and proxy lifecycle. Each
task's tests were run against a reference at that task's end state (560 to
638 tests, clippy clean); the reference is not in the repository. Adds the
runner-unavailable runbook entry and tip T23 (ETXTBSY in script tests).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/plans/M3b/files/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}");
+    }
+}