feat(manifest): lift git.user to the agent layer

Agents may declare git.user (name/email); it overlays the referenced
bottle's git.user per-field at Manifest.bottle_for (agent wins on
non-empty), mirroring the extends: merge. git.remotes is rejected on
agents — it carries credentials and host trust and stays bottle-only.

The overlay lives at bottle_for, the single chokepoint both backends
use, so the docker/smolmachines git provisioners are unchanged. Adds
Manifest.git_identity_summary with per-field (agent)/(bottle)
provenance, printed in both preflights and `info`.

Refs #94

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

bot_bottle/backend/docker/bottle_plan.py

@@ -85,6 +85,10 @@ class DockerBottlePlan(BottlePlan):
         print_multi("skills          ", list(agent.skills))
         info(f"bottle          : {agent.bottle}")
 
+        identity = manifest.git_identity_summary(spec.agent_name)
+        if identity:
+            info(f"  git identity  : {identity}")
+
         git_lines = [
             f"{u.upstream_host}:{u.upstream_port}"
             for u in self.git_gate_plan.upstreams