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).
