feat: hard-gate agent memory to OpenBrain via PreToolUse hook

Agents consistently ignore written instructions about memory routing
and default to writing local MEMORY.md files regardless of rules in
RUNTIME.md, CLAUDE.md, or MEMORY.md itself. Instructions alone are
insufficient — a technical gate is required.

Changes:
- Add tools/qa/prevent-memory-write.sh — PreToolUse hook that blocks
  Write/Edit/MultiEdit to ~/.claude/projects/*/memory/*.md (exit 2)
- Register hook in runtime/claude/settings.json PreToolUse array
- Update runtime/claude/RUNTIME.md: replace soft "Memory Override"
  note with hard-gate policy, what-goes-where table, and rationale
- Rewrite guides/MEMORY.md: OpenBrain as primary layer, blocked silos
  table, project continuity files, how-the-hook-works section

The correct behavior is now the only possible behavior for Claude Code.
All agent learnings route to OpenBrain where every harness can read them.

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

runtime/claude/settings.json

@@ -1,6 +1,18 @@
 {
   "model": "opus",
   "hooks": {
+    "PreToolUse": [
+      {
+        "matcher": "Write|Edit|MultiEdit",
+        "hooks": [
+          {
+            "type": "command",
+            "command": "~/.config/mosaic/tools/qa/prevent-memory-write.sh",
+            "timeout": 10
+          }
+        ]
+      }
+    ],
     "PostToolUse": [
       {
         "matcher": "Edit|MultiEdit|Write",