refactor(util): move is_ipv4_literal out of pipelock.py into util.py

The classifier is a pure dotted-quad regex check — nothing
pipelock-specific about it. Pipelock now imports it from util.
test_pipelock_classify.py retargets at the new location.

Two manifest-accessor functions in pipelock.py
(pipelock_bottle_allowlist, pipelock_bottle_ssh_hostnames) look
generic but are 1-line wrappers used only internally; they stay
for now.

claude_bottle/util.py

@@ -6,6 +6,7 @@ claude_bottle/backend/docker/util.py."""
 from __future__ import annotations
 
 import os
+import re
 
 
 def expand_tilde(path: str) -> str:
@@ -16,3 +17,15 @@ def expand_tilde(path: str) -> str:
         home = os.environ.get("HOME", "")
         return home + path[1:]
     return path
+
+
+_IPV4_RE = re.compile(r"^[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+$")
+
+
+def is_ipv4_literal(s: str) -> bool:
+    """True iff `s` looks like a dotted-quad IPv4 literal. Does not
+    validate octet ranges; consumers that care about that should run
+    a stricter check. Empty input returns False."""
+    if not s:
+        return False
+    return bool(_IPV4_RE.match(s))