move hive-sh4re's harness_dir() into hive-agent-sock, drop the dev-fallback derivation

nix/agent-modules/mcp.nix

@@ -261,9 +261,9 @@ in
         # injected via systemd.globalEnvironment by the meta flake
         # (set to /agents/<name>/harness and /agents/<name>/state
         # respectively). The daemon uses these to derive its task +
-        # loose-ends dir paths; without them it falls back to deriving
-        # harness/ as a sibling of state/, which produces the same
-        # value but is less robust if the two vars ever diverge.
+        # loose-ends dir paths; `hive_agent_sock::paths::harness_dir`
+        # panics loudly if HYPERHIVE_HARNESS_DIR is unset rather than
+        # deriving a fallback, since every service here always gets it.
       };
       serviceConfig = {
         ExecStart = "${config.hyperhive.packages.hive-bash-daemon}/bin/hive-bash-daemon --http 127.0.0.1:${toString config.hyperhive.mcp.bashHttpPort}";