Allow host-managed agent slice memory policy without runtime MemoryHigh override #24708

The Linux agentScope implementation unconditionally calls systemctl --user set-property --runtime app-t3code-agents.slice MemoryHigh= MemoryMax=. These generated user.control drop-ins outrank administrator-managed user drop-ins. There is no configuration seam in agentSliceMemoryLimits or the set-property call to respect a host-wide resource budget or disable MemoryHigh reclaim throttling. On a shared desktop this can conflict with independent inference/agent budgets; MemoryHigh reclaim can livelock indexing/tool workloads instead of enforcing a predictable hard ceiling. Please allow an explicit host-managed policy mode (do not generate these two runtime overrides), or configurable high/max values including high=infinity. Preserve per-thread scope lifecycle/OOM handling. Source: apps/server/src/process/agentScope.ts, agentSliceMemoryLimits and cached systemd initialization. Reproduction: install a user drop-in with MemoryHigh=infinity and a smaller hard MemoryMax, then initialize an agent; show properties and runtime user.control files demonstrate the override. No private host identifiers or workload data are needed to reproduce.