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_extends.py

@@ -71,7 +71,8 @@ def _merge_bottles(
     name: str,
 ) -> Bottle:
     """Apply PRD 0025 merge rules."""
-    from .manifest import Bottle, GitUser, _validate_egress_routes
+    from .manifest import Bottle, GitUser
+    from .manifest_egress import _validate_egress_routes
 
     # Parse the child's declared fields into a Bottle (with the
     # usual defaults for anything missing). Validation runs the same
@@ -122,7 +123,7 @@ def _merge_bottles(
 
 
 def _child_declares_git_gate_repos(child_raw: dict[str, object]) -> bool:
-    from .manifest import _as_json_object
+    from ._manifest_util import _as_json_object
 
     git_raw = child_raw.get("git-gate")
     if git_raw is None: