🏗️ 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/spire/.codex-plugin/plugin.json

@@ -1,7 +1,7 @@
 {
   "name": "spire",
   "version": "0.1.0",
-  "description": "Spire — an agent orchestration system for structured development. Session coordination (Igor), domain knowledge (adepts), dispatching (athanor), knowledge management (tome), workpad, research, and implementation governance.",
+  "description": "Spire — an agent orchestration system for structured development. Session coordination (Igor), domain knowledge (adepts), dispatching (athanor), workpad, research, and implementation governance. Knowledge navigation via the separate tome plugin (opt-in).",
   "author": {
     "name": "Gabor Körber",
     "email": "gab@g4b.org"