feat(manifest): replace git key with git-gate (PRD 0047)

- BOTTLE_KEYS and AGENT_KEYS_OPTIONAL: "git" → "git-gate"
- GitEntry: remove from_dict/from_remote_dict; add from_repos_entry
  parsing url/identity/host_key with repo name as the dict key
- GitUser.from_dict: error messages updated to git-gate.user
- _parse_git_config → _parse_git_gate_config; repos/user subkeys
- Bottle.from_dict: reads git-gate key; "git" key raises a migration error
- Agent.from_dict: reads git-gate key; repos rejected at agent level
- manifest_extends: _child_declares_git_remotes → _child_declares_git_gate_repos
- manifest_loader: threads git-gate frontmatter key into agent_dict

bot_bottle/manifest_loader.py

@@ -93,13 +93,13 @@ def load_agents_from_dir(
         validate_agent_frontmatter_keys(path, fm.keys())
         # Build the dict Agent.from_dict expects. The body becomes
         # prompt; Claude Code passthrough fields stay in fm and get
-        # ignored by Agent.from_dict (which reads bottle/skills/git/prompt).
+        # ignored by Agent.from_dict (reads bottle/skills/git-gate/prompt).
         agent_dict: dict[str, object] = {
             "bottle": fm.get("bottle"),
             "skills": fm.get("skills", []),
             "prompt": body.strip(),
         }
-        if "git" in fm:
-            agent_dict["git"] = fm["git"]
+        if "git-gate" in fm:
+            agent_dict["git-gate"] = fm["git-gate"]
         out[name] = Agent.from_dict(name, agent_dict, bottle_names)
     return out