🏗️ Extract tome from spire into standalone plugin

Tome is opt-in per project — each project declares its own knowledge
management approach. Spire retains a small knowledge-awareness skill
that explains what tome does and provides a fallback strategy for
projects without it.

- New plugins/tome/ with own manifests, README, all 5 sub-skills
- Spire keeps skills/knowledge/SKILL.md as awareness stub
- Updated all READMEs and marketplace JSONs
- tome: namespace refs now correct (tome is its own plugin)

.agents/plugins/marketplace.json

@@ -16,6 +16,18 @@
       },
       "category": "Productivity"
     },
+    {
+      "name": "tome",
+      "source": {
+        "source": "local",
+        "path": "./plugins/tome"
+      },
+      "policy": {
+        "installation": "AVAILABLE",
+        "authentication": "ON_INSTALL"
+      },
+      "category": "Productivity"
+    },
     {
       "name": "g4b_ai",
       "source": {