fix(smolmachines): fix CA provisioning and git-http startup on Linux

Two fixes for the Linux smolmachines backend:

- provision_ca: add mkdir -p before cp_in so the
  /usr/local/share/ca-certificates/ directory exists in the smolvm
  guest even when smolvm pack omits it (empty dir on Linux).
- git_http_backend: the fallback import in the except ImportError
  block tried `from git_gate import ...`, but git_gate.py is not
  deployed to the sidecar container and has its own package-relative
  imports; use the literal value 15 directly instead.

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

bot_bottle/agent_provider.py

@@ -227,6 +227,10 @@ class AgentProvider(ABC):
         from .backend.util import AGENT_CA_PATH, log_ca_fingerprint, select_ca_cert
         from .log import die
         cert_host_path, label = select_ca_cert(plan.egress_plan)
+        # Ensure the target directory exists. smolvm's pack step may not
+        # preserve the empty /usr/local/share/ca-certificates/ directory
+        # on Linux; mkdir -p is idempotent and safe for all backends.
+        bottle.exec("mkdir -p /usr/local/share/ca-certificates", user="root")
         bottle.cp_in(str(cert_host_path), AGENT_CA_PATH)
         r = bottle.exec(
             f"chmod 644 {AGENT_CA_PATH} && update-ca-certificates",