feat(manifest): git-gate.signing opt-in (PRD chunk 2)

Implements the manifest surface for the signed-commits & audit-attribution
PRD (#423, PR #480): a bottle opts into per-activation commit signing with

    git-gate:
      signing:
        enabled: true

- New ManifestGitSigning value type (frozen, enabled: bool = False) parsed
  under git-gate.signing; unknown sub-keys and non-bool `enabled` die with
  targeted errors. parse_git_gate_config now returns (repos, user, signing).
- ManifestBottle gains git_signing; defaults to off, so bottles that omit
  the block are unchanged.
- Bottle-only: git-gate.signing on an agent is rejected by the existing
  agent-level non-`user` guard (like git-gate.repos).
- extends/runtime merges thread git_signing with an enable-wins overlay
  (a child enabling signing wins; omitting inherits the parent), mirroring
  the git_user non-empty-wins overlay.
- Facade exports ManifestGitSigning.

Tests: tests/unit/test_manifest_git_signing.py (parse, validation,
agent-level rejection, extends overlay). Full unit suite green.

This is the first stacked chunk; signing pipeline, gate signature check,
and control-plane verification + audit tables follow per the PRD.

Issue: #423

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/manifest/__init__.py

@@ -66,7 +66,12 @@ if TYPE_CHECKING:
     from .agent import ManifestAgent, ManifestAgentProvider
     from .bottle import ManifestBottle
     from .egress import EGRESS_AUTH_SCHEMES, ManifestEgressConfig, ManifestEgressRoute
-    from .git import ManifestGitEntry, ManifestGitUser, ManifestKeyConfig
+    from .git import (
+        ManifestGitEntry,
+        ManifestGitSigning,
+        ManifestGitUser,
+        ManifestKeyConfig,
+    )
 
 
 # Facade name -> submodule that defines it. The aggregate model (`Manifest`,
@@ -82,6 +87,7 @@ _LAZY_MODULES: dict[str, str] = {
     "ManifestEgressRoute": "egress",
     "ManifestEgressConfig": "egress",
     "ManifestGitEntry": "git",
+    "ManifestGitSigning": "git",
     "ManifestGitUser": "git",
     "ManifestKeyConfig": "git",
 }
@@ -107,6 +113,7 @@ __all__ = [
     "ManifestIndex",
     "ManifestError",
     "ManifestGitEntry",
+    "ManifestGitSigning",
     "ManifestGitUser",
     "ManifestKeyConfig",
     "ManifestAgentProvider",