agents: read the memory cap at boot, not from the build

An agent's memory cap was rendered into its flake (`memoryMaxBytes` →
`services.hyperhive.agent.claudeMemoryMaxBytes`), so the closure depended
on it and a changed cap reached the JSC heap ceiling and the subagent
`MemoryHigh=` only after a rebuild.

hive-c0re now writes the effective `MemoryMax=` in bytes to the agent's
runtime dir (`/run/hive/memory-max-bytes` inside) in `write_dropins`,
from the same value the drop-in gets. The file is removed for
`infinity` or a RAM percentage, and left alone when the drop-in is kept.

Inside the container, `nix/agent-modules/memory-cap.nix` reads it at
boot: an environment generator sets `BUN_JSC_forceRAMSize` (75%) for
every unit and `machinectl shell`, `/etc/set-environment` exports it in
login shells, and a unit generator writes `MemoryHigh=` (2/3) as a
drop-in for `hive-subagent-daemon`.

The render drops `memoryMaxBytes`, the agent option is removed (both
spellings fail eval with a removal message), and
`effective_memory_bytes_from` goes with its only caller.

Part of #5081 (slice S0-2).

nix/script-tests/agent-memory-cap.nix

@@ -0,0 +1,42 @@
+# `checks.script-test-agent-memory-cap` — runs the three readers of
+# `/run/hive/memory-max-bytes` in ../agent-modules/memory-cap.nix: the
+# environment generator, the unit generator, and the login-shell snippet
+# `/etc/set-environment` sources. The cases are in ./agent-memory-cap.sh.
+#
+# What runs is the rendered scripts. The one edit is the cap file's path, moved
+# under the build directory, and the snippet's reference to the environment
+# generator, pointed at that edited copy.
+{
+  pkgs,
+  lib,
+  self,
+  nixosSystem,
+}:
+let
+  inherit
+    (import ../module-eval/lib.nix {
+      inherit
+        pkgs
+        lib
+        self
+        nixosSystem
+        ;
+    })
+    agentWith
+    ;
+
+  machine = agentWith { };
+in
+pkgs.runCommand "hyperhive-script-test-agent-memory-cap"
+  {
+    ENV_GENERATOR =
+      machine.environment.etc."systemd/system-environment-generators/50-hyperhive-memory-cap".source;
+    UNIT_GENERATOR = machine.systemd.generators.hyperhive-memory-cap;
+    PROFILE = machine.environment.etc."hyperhive/memory-cap.sh".source;
+    EXTRA_INIT = machine.environment.extraInit;
+    SHELLS = "${pkgs.dash}/bin/dash ${pkgs.bash}/bin/sh";
+  }
+  ''
+    ${pkgs.bash}/bin/bash ${./agent-memory-cap.sh}
+    touch "$out"
+  ''