refactor(bottles): introduce BottlePlan base + move print onto plan

- Add BottlePlan (frozen dataclass + ABC) with spec, stage_dir, and an
  abstract `print(*, remote_control)` method.
- DockerBottlePlan now inherits from BottlePlan; spec/stage_dir come
  from the base, Docker-specific fields stay on the subclass.
- Move BottleSpec from bottles/docker.py to bottles/__init__.py so the
  cross-platform types live together. docker.py pulls them via
  `from . import ...`.
- Move show_plan from cli/start.py to `DockerBottlePlan.print`. Caller
  becomes `plan.print(remote_control=...)`. The CLI no longer reads
  any Docker-specific fields.
- BottlePlatform.prepare is now typed `Callable[..., BottlePlan]`.

cmd_start drops ~46 more lines.

claude_bottle/bottles/docker.py

@@ -34,7 +34,7 @@ from .. import skills as skills_mod
 from .. import ssh as ssh_mod
 from ..env_resolve import env_resolve
 from ..log import die, info
-from ..manifest import Manifest
+from . import BottlePlan, BottleSpec
 
 
 # --- Runtime detection -----------------------------------------------------
@@ -51,36 +51,20 @@ def runsc_available() -> bool:
     return r.returncode == 0 and "runsc" in r.stdout
 
 
-# --- Spec + Plan -----------------------------------------------------------
+# --- Plan ------------------------------------------------------------------
 
 
 @dataclass(frozen=True)
-class BottleSpec:
-    """CLI-supplied intent. Platform-agnostic — each platform's prepare
-    step consumes it and produces its own platform-specific plan.
-    Resolved values (image names, container name, scratch paths, runsc
-    availability) live on the plan, not the spec."""
+class DockerBottlePlan(BottlePlan):
+    """Docker-specific resolved fields produced by prepare_docker_bottle.
+    Inherits `spec` and `stage_dir` from BottlePlan."""
 
-    manifest: Manifest
-    agent_name: str
-    copy_cwd: bool
-    user_cwd: str
-    forward_oauth_token: bool
-
-
-@dataclass(frozen=True)
-class DockerBottlePlan:
-    """Output of prepare_docker_bottle. Frozen; the launch step consumes
-    it without further resolution. show_plan reads from it directly."""
-
-    spec: BottleSpec
     slug: str
     container_name: str
     container_name_pinned: bool
     image: str
     derived_image: str           # "" -> no derived image
     runtime_image: str           # image actually launched (derived or base)
-    stage_dir: Path
     env_file: Path
     args_file: Path
     prompt_file: Path
@@ -89,6 +73,47 @@ class DockerBottlePlan:
     allowlist_summary: str
     use_runsc: bool
 
+    def print(self, *, remote_control: bool) -> None:
+        """Render the y/N preflight summary to stderr. Pure presentation."""
+        spec = self.spec
+        manifest = spec.manifest
+        agent = manifest.agents[spec.agent_name]
+        bottle = manifest.bottle_for(spec.agent_name)
+
+        env_names = list(bottle.env.keys())
+        if spec.forward_oauth_token:
+            env_names.append("CLAUDE_CODE_OAUTH_TOKEN")
+
+        ssh_hosts = [e.Host for e in bottle.ssh]
+        prompt_first_line = agent.prompt.splitlines()[0] if agent.prompt else ""
+        runtime_label = "runsc (gVisor)" if self.use_runsc else "runc (default)"
+
+        print(file=sys.stderr)
+        info(f"agent           : {spec.agent_name}")
+        info(f"image           : {self.image}")
+        if self.derived_image:
+            info(
+                f"cwd             : {spec.user_cwd} -> /home/node/workspace "
+                f"(derived: {self.derived_image})"
+            )
+        info(f"container       : {self.container_name}")
+        info(f"stage dir       : {self.stage_dir}")
+        info("env (names only): " + (", ".join(env_names) if env_names else "(none)"))
+        info("skills          : " + (" ".join(agent.skills) if agent.skills else "(none)"))
+        info(f"docker runtime  : {runtime_label}")
+        info(f"bottle             : {agent.bottle}")
+        if ssh_hosts:
+            info(f"  ssh hosts     : {', '.join(ssh_hosts)}")
+        else:
+            info("  ssh hosts     : (none)")
+        info(f"  egress        : {self.allowlist_summary}")
+        info(
+            f"prompt          : {len(agent.prompt)} chars; "
+            f"first line: {prompt_first_line or '(empty)'}"
+        )
+        info("remote-control  : " + ("enabled" if remote_control else "disabled"))
+        print(file=sys.stderr)
+
 
 # --- Bottle handle ---------------------------------------------------------