🏗️ 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)

plugins/tome/.claude-plugin/plugin.json

@@ -0,0 +1,9 @@
+{
+  "name": "tome",
+  "version": "0.1.0",
+  "description": "Tome — knowledge layer for agent-managed projects. Navigate, curate, check, handoff, init. Opt-in per project: install the plugin, then run /tome:init to set up the knowledge structure.",
+  "author": {
+    "name": "Gabor Körber",
+    "email": "gab@g4b.org"
+  }
+}