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_schema.py

@@ -16,10 +16,10 @@ _FILENAME_RX = re.compile(r"^[a-z][a-z0-9-]*$")
 # sets dies with a "did you mean" pointer: typos should not silently
 # ghost into an empty config.
 BOTTLE_KEYS = frozenset(
-    {"env", "extends", "agent_provider", "git", "egress", "supervise"}
+    {"env", "extends", "agent_provider", "git-gate", "egress", "supervise"}
 )
 AGENT_KEYS_REQUIRED = frozenset({"bottle"})
-AGENT_KEYS_OPTIONAL = frozenset({"skills", "git"})
+AGENT_KEYS_OPTIONAL = frozenset({"skills", "git-gate"})
 
 # Claude Code subagent fields bot-bottle ignores at launch but does
 # not reject. This lets the same file double as