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>

AGENTS.md

@@ -14,8 +14,9 @@ Before responding to any user message, you MUST read these files in order:
 4. `~/.config/mosaic/AGENTS.md`
 5. `~/.config/mosaic/TOOLS.md`
 6. `~/.config/mosaic/guides/E2E-DELIVERY.md`
-7. Project-local `AGENTS.md` (if present)
-8. Runtime-specific reference:
+7. `~/.config/mosaic/guides/MEMORY.md`
+8. Project-local `AGENTS.md` (if present)
+9. Runtime-specific reference:
    - Claude: `~/.config/mosaic/runtime/claude/RUNTIME.md`
    - Codex: `~/.config/mosaic/runtime/codex/RUNTIME.md`
    - OpenCode: `~/.config/mosaic/runtime/opencode/RUNTIME.md`
@@ -111,7 +112,6 @@ Load additional guides when the task requires them.
 | QA and test strategy | `~/.config/mosaic/guides/QA-TESTING.md` |
 | Secrets and vault usage | `~/.config/mosaic/guides/VAULT-SECRETS.md` |
 | Orchestrator estimation heuristics | `~/.config/mosaic/guides/ORCHESTRATOR-LEARNINGS.md` |
-| Shared memory patterns | `~/.config/mosaic/guides/MEMORY.md` |
 
 ## Embedded Delivery Cycle (Hard Rule)