fix: enforce durable memory in mosaic memory dir, not runtime-native silos

MEMORY.md was conditionally loaded so agents defaulted to their native
memory locations (e.g. ~/.claude/projects/*/memory/). This caused durable
learnings to be siloed per-runtime instead of shared across agents.

- Move MEMORY.md to mandatory load order in AGENTS.md (position 7)
- Add Memory Override section to all three runtime configs (Claude, Codex,
  OpenCode) explicitly forbidding native memory silos for durable data
- Add memory/ directory with .gitkeep to bootstrap source
- Add mkdir -p for memory/ in install.sh post-sync step

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

install.sh

@@ -138,6 +138,9 @@ fi
 
 sync_framework
 
+# Ensure memory directory exists (preserved across upgrades, may not exist on fresh install)
+mkdir -p "$TARGET_DIR/memory"
+
 chmod +x "$TARGET_DIR"/bin/*
 chmod +x "$TARGET_DIR"/install.sh