feat: add mosaic-framework OpenClaw plugin + install-oc-plugins command

- oc-plugins/mosaic-framework/ — plugin that mechanically injects Mosaic
  framework rails into every OpenClaw agent session and ACP worker spawn
  - before_agent_start: appendSystemContext (static rules, cached) +
    prependContext (dynamic mission state per agent project root)
  - subagent_spawning: writes ~/.codex/instructions.md before Codex binary
    starts; no mission context (workers detect own CWD mission)
  - Optional gate: requireMission blocks ACP spawns with no active mission

Install: mosaic install-oc-plugins --project-root ~/src/<repo>

oc-plugins/mosaic-framework/package.json

@@ -0,0 +1,15 @@
+{
+  "name": "mosaic-framework",
+  "version": "0.1.0",
+  "type": "module",
+  "main": "index.ts",
+  "description": "Injects Mosaic framework rails, runtime contract, and active mission context into all OpenClaw agent sessions and ACP subagent spawns.",
+  "openclaw": {
+    "extensions": [
+      "./index.ts"
+    ]
+  },
+  "devDependencies": {
+    "openclaw": "*"
+  }
+}