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/cli/info.py

@@ -31,6 +31,9 @@ def cmd_info(argv: list[str]) -> int:
         f"first line: {prompt_first_line or '(empty)'}"
     )
     info(f"bottle             : {agent.bottle}")
+    identity = manifest.git_identity_summary(args.name)
+    if identity:
+        info(f"  git identity  : {identity}")
     if bottle.git:
         for e in bottle.git:
             info(