docs: add CONTRIBUTING.md defining read/write governance for all agents

- Who owns which section (scope rules per agent)
- What to save vs never save (no secrets, no session chatter)
- Mandatory conventions (frontmatter, wikilinks, auto-generated pages)
- Git workflow for contributing agents
- Freshness responsibility: each agent keeps its own section current

Link CONTRIBUTING from index.md start-here block.

index.md

@@ -5,6 +5,7 @@
 ## Start here
 - [[current-state]] — Current systems status
 - [[agents/index]] — Agent estate map (start here for "what runs where")
+- [[CONTRIBUTING]] — **How agents read/write this wiki (read this first)**
 - [[systems/hermes-agent]] — Hermes Agent overview and version
 - [[systems/current-profile]] — Active Hermes profile details