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_util.py

@@ -0,0 +1,24 @@
+"""Shared manifest primitives used by all manifest sub-modules."""
+
+from __future__ import annotations
+
+from typing import cast
+
+
+class ManifestError(Exception):
+    """A manifest file (or the manifest tree) is invalid."""
+
+
+def _as_json_object(value: object, label: str) -> dict[str, object]:
+    """Assert that `value` is a JSON object (str-keyed dict) and return
+    a view typed as `dict[str, object]` so downstream `.get(...)` calls
+    have a typed surface."""
+    if not isinstance(value, dict):
+        raise ManifestError(f"{label} must be a JSON object (was {type(value).__name__})")
+    items = cast(dict[object, object], value)
+    out: dict[str, object] = {}
+    for k, v in items.items():
+        if not isinstance(k, str):
+            raise ManifestError(f"{label} keys must be strings (found {type(k).__name__})")
+        out[k] = v
+    return out