🏗️ 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)

.claude-plugin/marketplace.json

@@ -8,7 +8,12 @@
     {
       "name": "spire",
       "source": "./plugins/spire",
-      "description": "Agent orchestration system: session coordinator (Igor), domain knowledge agents (adepts), dispatching orchestrator (athanor), knowledge layer (tome), workpad, research tools"
+      "description": "Agent orchestration system: session coordinator (Igor), domain knowledge agents (adepts), dispatching orchestrator (athanor), workpad, research tools"
+    },
+    {
+      "name": "tome",
+      "source": "./plugins/tome",
+      "description": "Knowledge layer: navigate, curate, check, handoff, init. Opt-in per project, pairs with spire."
     },
     {
       "name": "g4b_ai",