feat: add Tier 0 memory overlay for low-RAM machines (Light-Heart-Labs#286)

* feat: add Tier 0 memory overlay for low-RAM machines

Default service reservations total 7.6GB which exceeds Tier 0
hardware (<8GB). Adds docker-compose.tier0.yml overlay that reduces
reservations to 2.4GB total — a 5.2GB reduction — so services can
coexist with the Qwen3.5-2B model on 4-7GB machines.

Overlay is automatically layered when TIER=0 via compose-select.sh.
Services get lower limits but Docker's OOM killer handles pressure
gracefully. No services are removed — full stack on any hardware.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* feat: include Tier 0 memory overlay in macOS installer

The macOS installer builds its own COMPOSE_FLAGS independently from
compose-select.sh. Without this, macOS Tier 0 machines would still
get the default 7.6GB memory reservations.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: Lightheartdevs