feat(init): create /var/lib/decnet/artifacts with setgid + group-write

DEBT-035 step 2. Today the artifacts subtree is auto-created by
Docker as root when a decoy container's bind-mount fires for the
first time. The resulting permissions are root:root 0o755 — the API
process (running as the decnet user) hits PermissionError trying to
read transcripts written by the container, and the soft-fail 404
path gets exercised on every fresh deploy.

Add `/var/lib/decnet/artifacts` to init's dirs list with mode 0o2775:

* 0o2000 — setgid bit. New files inherit the directory's group
  (decnet), regardless of which uid created them. This is the load-
  bearing bit for cross-container reads.
* 0o0775 — owner+group rwx, world rx. Group-write lets the API
  process and the local TTP worker read each other's outputs
  without a manual chown.

`_ensure_dir` already respects the full mode word via `os.chmod`,
no helper change needed.

Test asserts the resulting directory carries exactly 0o2775 after
a fresh `decnet init --prefix`. Defence-in-depth: this works even
if the per-decoy compose `user:` directive (next commit) misses a
template — files still land in the decnet group.

tests/cli/test_init.py

@@ -241,6 +241,27 @@ def test_init_persists_api_user_group_to_decnet_ini(
     assert "api-group = decoygrp" in body
 
 
+def test_init_creates_artifacts_dir_with_setgid_and_group_write(
+    monkeypatch: Any, tmp_path: Path, subprocess_calls: List[List[str]],
+    no_missing_tools: None, missing_user_and_group: None,
+) -> None:
+    """DEBT-035: `/var/lib/decnet/artifacts` must come up with
+    setgid (0o2000) + group-write (0o0070) so every file written
+    by a decoy container inherits the decnet group, regardless of
+    which container's uid did the creating."""
+    _seed_deploy(monkeypatch, tmp_path)
+    prefix = tmp_path / "root"
+    r = runner.invoke(app, ["init", "--no-start", "--prefix", str(prefix)])
+    assert r.exit_code == 0, r.output
+
+    artifacts = prefix / "var/lib/decnet/artifacts"
+    assert artifacts.is_dir(), "artifacts dir must be created at init time"
+    mode = artifacts.stat().st_mode & 0o7777
+    assert mode == 0o2775, (
+        f"expected 0o2775 (setgid + 0o775), got {oct(mode)}"
+    )
+
+
 def test_init_decnet_ini_loads_via_config_ini(
     monkeypatch: Any, tmp_path: Path, subprocess_calls: List[List[str]],
     no_missing_tools: None, missing_user_and_group: None,