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/backend/docker/prepare.py

@@ -15,7 +15,7 @@ from datetime import datetime, timezone
 from dataclasses import replace
 from pathlib import Path
 
-from ...agent_provider import agent_provision_plan, runtime_for
+from ...agent_provider import PROVIDER_TEMPLATES, agent_provision_plan, runtime_for
 from ...egress import Egress
 from ...env import ResolvedEnv, resolve_env
 from ...git_gate import GitGate
@@ -100,6 +100,15 @@ def resolve_plan(
     elif provider_runtime.dockerfile:
         image_default = provider_runtime.image
         dockerfile_path = provider_runtime.dockerfile
+    elif provider.template not in PROVIDER_TEMPLATES:
+        user_dockerfile = (
+            Path.home() / ".bot-bottle" / "contrib" / provider.template / "Dockerfile"
+        )
+        if user_dockerfile.is_file():
+            image_default = f"bot-bottle-{provider.template}:{slug}"
+            dockerfile_path = str(user_dockerfile)
+        else:
+            image_default = provider_runtime.image
     else:
         image_default = provider_runtime.image
     image = os.environ.get("BOT_BOTTLE_IMAGE", image_default)