Add the list of predefined keys, and refuse removing a key a module still names

Mesh and node config may hold values only for keys on a fixed, reviewed list (hq ADR 0348 clause 7, design 57 package K2, issue 661). predefined.json at the root starts with font-size (a number of points, 6 to 72) and agent-managed-settings (a trusted object). The merge check refuses a change that removes a key a manifest still names, against the trunk's list at the merge base; whether a removed key still holds a value is mesh-config's to refuse (package K3). Part 1 of 2.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

predefined.json

@@ -0,0 +1,13 @@
+{
+  "agent-managed-settings": {
+    "type": "object",
+    "description": "The managed settings every agent session on a node is held to (Claude Code's managed-settings.json), as a JSON object.",
+    "trusted": true
+  },
+  "font-size": {
+    "type": "number",
+    "description": "The size a desktop program shows its text in, in points.",
+    "minimum": 6,
+    "maximum": 72
+  }
+}