refactor: split manifest.py into domain-specific modules

Closes #157. Distributes the 1,026-line manifest.py across four
focused modules:

- _manifest_util.py: ManifestError + _as_json_object (shared base)
- manifest_git.py: GitEntry, GitUser, git-gate config helpers
- manifest_egress.py: EgressRoute, EgressConfig, PipelockRoutePolicy
- manifest_agent.py: AgentProvider, Agent

manifest.py is now the residual orchestration layer: Bottle, Manifest,
and re-exports of all public names so existing callers are unaffected.
All 867 unit tests pass.

bot_bottle/manifest_schema.py

@@ -58,7 +58,7 @@ def _validate_frontmatter_keys(
     keys: object,
     allowed_keys: frozenset[str],
 ) -> None:
-    from .manifest import ManifestError
+    from ._manifest_util import ManifestError
 
     key_set = set(keys)
     unknown = key_set - allowed_keys