refactor(backend): extract shared print_multi for plan preflights

Addresses PR #62 review comments on
claude_bottle/backend/smolmachines/bottle_plan.py:

- Lift the multi-value label printer (was a nested helper inside
  DockerBottlePlan.print) into a new module
  claude_bottle/backend/print_util.py:print_multi. Both backends
  use it for env / skills / git / egress lines.

- Strip the three smolmachines-preflight lines the review flagged:
  the gvproxy subnet line, the smolfile path line, and the
  gvproxy-config path line. Internal detail — operators see the
  agent / env / skills / bottle / git / egress that already
  matter on the docker side, and nothing else.

- Add `git → upstream` to the smolmachines git output to match
  what's useful at preflight time (the docker version shows
  upstream_host:port; this is similar shape).

Leaves the slug=spec.identity-or-mint pattern alone pending a
reply on PR comment #432 — the docker backend uses the same
pattern to preserve identity across `resume`, so dropping it
would silently break the resume path once smolmachines launch
lands.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

claude_bottle/backend/docker/bottle_plan.py

@@ -17,6 +17,7 @@ from ...log import info
 from ...pipelock import PipelockProxyPlan
 from ...supervise import SupervisePlan
 from .. import BottlePlan
+from ..print_util import print_multi
 
 
 @dataclass(frozen=True)
@@ -70,22 +71,10 @@ class DockerBottlePlan(BottlePlan):
         # from the agent to the proxy is needed.
         env_names = sorted(set(bottle.env.keys()) | set(self.forwarded_env.keys()))
 
-        def _multi(label: str, values: list[str]) -> None:
-            """Print a label with N continuation-indented values. Used
-            for env / skills / git-gate / egress where one item
-            per line keeps the summary scannable."""
-            if not values:
-                info(f"{label}: (none)")
-                return
-            info(f"{label}: {values[0]}")
-            indent = " " * (len(label) + 2)
-            for v_ in values[1:]:
-                info(f"{indent}{v_}")
-
         print(file=sys.stderr)
         info(f"agent           : {spec.agent_name}")
-        _multi("env             ", env_names)
-        _multi("skills          ", list(agent.skills))
+        print_multi("env             ", env_names)
+        print_multi("skills          ", list(agent.skills))
         info(f"bottle          : {agent.bottle}")
 
         git_lines = [
@@ -93,13 +82,13 @@ class DockerBottlePlan(BottlePlan):
             for u in self.git_gate_plan.upstreams
         ]
         if git_lines:
-            _multi("  git gate      ", git_lines)
+            print_multi("  git gate      ", git_lines)
 
         if self.egress_plan.routes:
             egress_lines = []
             for r in self.egress_plan.routes:
                 auth = f" [auth:{r.auth_scheme}]" if r.auth_scheme else ""
                 egress_lines.append(f"{r.host}{auth}")
-            _multi("  egress        ", egress_lines)
+            print_multi("  egress        ", egress_lines)
         print(file=sys.stderr)