agents: always write the memory cap, infinity for none

The cap file was removed for a cap of `infinity` or a RAM percentage,
so the boot check warned on every start of such an agent, for a
legitimate config. The file is now written on every start:

- a size: its byte count, as before;
- `infinity`: the literal `infinity`. The generators set no JSC heap
  ceiling and no subagent MemoryHigh=, and the boot check is quiet;
- a percentage: resolved on the host the way systemd resolves
  MemoryMax=N% for the container@ unit. That is permyriad (at most two
  decimals, at most 100%) of sysconf(_SC_PHYS_PAGES) *
  sysconf(_SC_PAGESIZE), rounded down to a page;
- a value systemd ignores (zero, over 100%, more than two decimals,
  unparseable) leaves the unit uncapped, so it is `infinity`.

The file is missing only when hive-c0re can't tell the cap: the
limits file and the kept drop-in are both unreadable, or a percentage
meets an unknown host memory size. The boot check now warns only for
a missing file or a first line that is neither a byte count nor
`infinity`.

Part of #5081 (slice S0-2).

docs/agent-lifecycle/persistence.md

@@ -404,7 +404,7 @@ Contents:
   `lifecycle::write_dropins` generates the systemd drop-in, **not** injected
   into the container — these are host-side caps on the container, so
   the capped party never sets them. It sees only the resulting memory cap,
-  as a byte count in its runtime dir (below). Fallback is per *field*:
+  as a byte count or `infinity` in its runtime dir (below). Fallback is per *field*:
   an absent file, absent agent, or absent field falls back to the
   hive-wide `services.hyperhive.c0re.agentCpuQuota` / `agentMemoryMax`,
   so an agent can override only its memory and still track the hive
@@ -510,8 +510,9 @@ containers can't start.
 - `/run/hyperhive/agents/<name>/mcp.sock` — per-agent socket
   (bind-mounted into the container as `/run/hive/mcp.sock`).
 - `/run/hyperhive/agents/<name>/memory-max-bytes` — the agent's effective
-  `MemoryMax=` in bytes, written by `lifecycle::write_dropins` before every
-  start (absent when the cap is `infinity` or a RAM percentage). The
+  `MemoryMax=` as a byte count (a percentage resolved against the host's
+  physical memory) or `infinity`, written by `lifecycle::write_dropins`
+  before every start. Absent only when hive-c0re can't tell the cap. The
   container reads it at boot as `/run/hive/memory-max-bytes`
   (`nix/agent-modules/memory-cap.nix`).