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/agent-modules/default.nix

@@ -41,6 +41,11 @@ let
     which the hive writes into every container, so no agent build sets it.
     Remove the setting.
   '';
+  memoryMaxBytesRemoved = ''
+    An agent reads its memory cap at boot from /run/hive/memory-max-bytes,
+    which the hive writes before every start, so no agent build sets it.
+    Remove the setting.
+  '';
 in
 {
   imports = [
@@ -60,6 +65,7 @@ in
     ./logs.nix
     ./matrix.nix
     ./mcp.nix
+    ./memory-cap.nix
     ./network.nix
     ./otel.nix
     ./packages.nix
@@ -94,6 +100,13 @@ in
     # Both spellings, for the reason given above.
     (lib.mkRemovedOptionModule [ "hyperhive" "hiveName" ] hiveNameRemoved)
     (lib.mkRemovedOptionModule [ "services" "hyperhive" "agent" "hiveName" ] hiveNameRemoved)
+    (lib.mkRemovedOptionModule [ "hyperhive" "claudeMemoryMaxBytes" ] memoryMaxBytesRemoved)
+    (lib.mkRemovedOptionModule [
+      "services"
+      "hyperhive"
+      "agent"
+      "claudeMemoryMaxBytes"
+    ] memoryMaxBytesRemoved)
   ];
 
   options.services.hyperhive.agent.icon = lib.mkOption {