feat(agent-provider): user plugin discovery, Dockerfile cascade, and provider-owned ca/git provisioning

- Add _load_user_plugin: loads AgentProvider subclass from
  ~/.bot-bottle/contrib/<name>/agent_provider.py; get_provider()
  checks there first before falling back to built-ins
- Add Dockerfile cascade to docker prepare: per-bottle override →
  manifest dockerfile → user plugin Dockerfile → provider default
- Move provision_ca and provision_git from backend-specific
  provision/ modules to AgentProvider ABC as overridable defaults;
  delete docker/provision/ca.py, docker/provision/git.py,
  smolmachines/provision/ca.py, smolmachines/provision/git.py
- Add git_gate_insteadof_host/scheme properties to BottlePlan base;
  SmolmachinesBottlePlan overrides them to return agent_git_gate_host
  and "http" so provision_git works correctly on both backends
- Move SIGKILL retry from smolmachines provision/ca.py into
  SmolmachinesBottle.exec via _exec_raw helper — all exec calls
  on smolmachines now transparently retry once on exit 137
- Relax manifest_agent template validation to allow user-defined
  template names; keep auth_token/forward_host_credentials guards
  for built-in-only features
- Update tests: rewrite test_docker_provision_git_user and
  test_smolmachines_provision to call provider methods directly;
  add TestSmolmachinesBottleExec for SIGKILL retry coverage

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bot_bottle/manifest_agent.py

@@ -49,11 +49,6 @@ class AgentProvider:
                 f"bottle '{bottle_name}' agent_provider.template must be a "
                 f"non-empty string"
             )
-        if template not in PROVIDER_TEMPLATES:
-            raise ManifestError(
-                f"bottle '{bottle_name}' agent_provider.template {template!r} "
-                f"is not one of {', '.join(sorted(PROVIDER_TEMPLATES))}"
-            )
         dockerfile = d.get("dockerfile", "")
         if not isinstance(dockerfile, str):
             raise ManifestError(
@@ -66,6 +61,12 @@ class AgentProvider:
                 f"bottle '{bottle_name}' agent_provider.auth_token must be a "
                 f"string (was {type(auth_token).__name__})"
             )
+        if auth_token and template not in PROVIDER_TEMPLATES:
+            raise ManifestError(
+                f"bottle '{bottle_name}' agent_provider.auth_token is only "
+                f"supported for built-in templates "
+                f"({', '.join(sorted(PROVIDER_TEMPLATES))})"
+            )
         if auth_token and template != "claude":
             raise ManifestError(
                 f"bottle '{bottle_name}' agent_provider.auth_token is only "
@@ -77,6 +78,12 @@ class AgentProvider:
                 f"bottle '{bottle_name}' agent_provider.forward_host_credentials "
                 f"must be a boolean (was {type(forward_host_credentials).__name__})"
             )
+        if forward_host_credentials and template not in PROVIDER_TEMPLATES:
+            raise ManifestError(
+                f"bottle '{bottle_name}' agent_provider.forward_host_credentials "
+                f"is only supported for built-in templates "
+                f"({', '.join(sorted(PROVIDER_TEMPLATES))})"
+            )
         if forward_host_credentials and template != "codex":
             raise ManifestError(
                 f"bottle '{bottle_name}' agent_provider.forward_host_credentials "