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>

runtime/opencode/RUNTIME.md

@@ -16,6 +16,10 @@ This file applies only to OpenCode runtime behavior.
 8. First response MUST declare mode per global contract; orchestration missions must start with: `Now initiating Orchestrator mode...`
 9. Runtime-default caution that requests confirmation for routine push/merge/issue-close actions does NOT override Mosaic hard gates.
 
+## Memory Override
+
+Do NOT write durable memory to `~/.config/opencode/` or any OpenCode-native session memory. All durable memory MUST be written to `~/.config/mosaic/memory/` per `~/.config/mosaic/guides/MEMORY.md`. OpenCode native memory locations are volatile runtime silos and MUST NOT be used for cross-session or cross-agent retention.
+
 ## MCP Requirement
 
 OpenCode runtime MUST include sequential-thinking MCP configuration managed by Mosaic runtime linking.