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/print_util.py

@@ -0,0 +1,28 @@
+"""Shared print helpers for BottlePlan.print implementations.
+
+Lifts the multi-value label printer out of DockerBottlePlan so the
+smolmachines backend (and any future backend) renders the same
+two-column scannable preflight without duplicating the indent
+math."""
+
+from __future__ import annotations
+
+from typing import Sequence
+
+from ..log import info
+
+
+def print_multi(label: str, values: Sequence[str]) -> None:
+    """Print `label: <value>` with continuation lines indented to
+    align under the first value. Empty `values` renders `(none)`.
+
+    Used by every backend's `BottlePlan.print` for env / skills /
+    git / egress — one item per line keeps the preflight summary
+    scannable when an agent has many of any of these."""
+    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}")