refactor(pipelock): prepare_proxy returns a ProxyPlan

Add a frozen ProxyPlan dataclass to pipelock.py (currently one field:
yaml_path; kept as a class so future proxy-level state has a home).

  - prepare_proxy(spec, stage_dir) now returns pipelock.ProxyPlan
    instead of a raw Path.
  - DockerBottlePlan replaces pipelock_yaml_path + pipelock_yaml_filename
    with a single proxy: ProxyPlan field.
  - launch reads plan.proxy.yaml_path.parent / .name when calling
    pipelock_start. Eventually pipelock_start should just take a Path
    but that's a separate change.

claude_bottle/pipelock.py

@@ -15,6 +15,7 @@ from __future__ import annotations
 import os
 import re
 import subprocess
+from dataclasses import dataclass
 from pathlib import Path
 
 from .log import die, info, warn
@@ -123,6 +124,15 @@ def pipelock_allowlist_summary(manifest: Manifest, bottle_name: str) -> str:
 # --- Proxy class -----------------------------------------------------------
 
 
+@dataclass(frozen=True)
+class ProxyPlan:
+    """Output of a proxy's prepare step; consumed by launch when the
+    proxy needs to be brought up. Currently single-field (the on-host
+    yaml path); kept as a dataclass so future proxy state has a home."""
+
+    yaml_path: Path
+
+
 class PipelockProxy:
     """The pipelock egress proxy. Encapsulates the YAML-config
     generation (and is the natural home for any future proxy-level