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/smolmachines/backend.py

@@ -22,8 +22,6 @@ from . import smolvm as _smolvm
 from .bottle import SmolmachinesBottle
 from .bottle_cleanup_plan import SmolmachinesBottleCleanupPlan
 from .bottle_plan import SmolmachinesBottlePlan
-from .provision import ca as _ca
-from .provision import git as _git
 from .provision import workspace as _workspace
 
 
@@ -55,21 +53,11 @@ class SmolmachinesBottleBackend(
         with _launch.launch(plan, provision=self.provision) as bottle:
             yield bottle
 
-    def provision_ca(
-        self, plan: SmolmachinesBottlePlan, bottle: Bottle
-    ) -> None:
-        _ca.provision_ca(plan, bottle)
-
     def provision_workspace(
         self, plan: SmolmachinesBottlePlan, bottle: Bottle
     ) -> None:
         _workspace.provision_workspace(plan, bottle)
 
-    def provision_git(
-        self, plan: SmolmachinesBottlePlan, bottle: Bottle
-    ) -> None:
-        _git.provision_git(plan, bottle)
-
     def supervise_mcp_url(self, plan: SmolmachinesBottlePlan) -> str:
         """The smolmachines guest reaches the supervise sidecar via a
         host-published random port the launch step pinned earlier