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.
