docs(manifest): document + demo agent-level git.user

README manifest section documents the agent git.user overlay, the
bottle-only git.remotes boundary, and the claimed-not-vouched trust
note. Collapses the example: implementer carries its own identity
against the shared dev bottle instead of an identity-only bottle.

Refs #94

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

examples/agents/implementer.md

@@ -5,6 +5,10 @@ model: opus
 bottle: dev
 skills:
   - init-prd
+git:
+  user:
+    name: implementer-bot
+    email: eric+implementer@dideric.is
 ---
 
 You are a feature-implementation agent running inside an ephemeral