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.

@@ -73,2 +73,3 @@
     """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

@@ -0,0 +15,4 @@
+EGRESS_AUTH_SCHEMES = ("Bearer", "token")
+
+
+def _is_ip_literal(value: str) -> bool:

@@ -0,0 +6,4 @@
+from dataclasses import dataclass, field
+from typing import cast
+
+from ._manifest_util import ManifestError, _as_json_object

@@ -0,0 +9,4 @@
+    """A manifest file (or the manifest tree) is invalid."""
+
+
+def _as_json_object(value: object, label: str) -> dict[str, object]:

@@ -0,0 +44,4 @@
+    return (user, host, port, path)
+
+
+def _validate_unique_git_names(bottle_name: str, git: tuple[GitEntry, ...]) -> None:

@@ -0,0 +15,4 @@
+    return value
+
+
+def _parse_git_upstream(url: str, label: str) -> tuple[str, str, str, str]:

@@ -0,0 +180,4 @@
+        return not self.name and not self.email
+
+
+def _parse_git_gate_config(