refactor: rename platform abstraction to backend

Across the package:
  - claude_bottle/platform/         -> claude_bottle/backend/
  - platform/docker/platform.py     -> backend/docker/backend.py
  - class BottlePlatform            -> BottleBackend
  - class DockerBottlePlatform      -> DockerBottleBackend
  - get_bottle_platform()           -> get_bottle_backend()
  - env var CLAUDE_BOTTLE_PLATFORM  -> CLAUDE_BOTTLE_BACKEND
  - dict _PLATFORMS                 -> _BACKENDS

"Backend" is shorter and more established as the term for a
pluggable strategy-pattern implementation. "Platform" was vague
(could mean OS, hardware, cloud) and mildly redundant — Docker is
itself a platform.

The previous PRD section claiming "the Backend protocol was
rejected" referred to a low-level run/exec/cp/network_connect
protocol; the name was never the reason. The PRD is updated to
describe that rejected design by shape rather than by name.

The bottle/agent concepts and the manifest schema are unchanged.

claude_bottle/backend/docker/bottle_cleanup_plan.py

@@ -0,0 +1,36 @@
+"""DockerBottleCleanupPlan — concrete subclass of BottleCleanupPlan.
+
+Holds the tuples of container and network names that
+DockerBottleBackend.cleanup will remove. The y/N preflight reads
+these via `print`; the CLI short-circuits via `empty`.
+"""
+
+from __future__ import annotations
+
+import sys
+from dataclasses import dataclass
+
+from ...log import info
+from .. import BottleCleanupPlan
+
+
+@dataclass(frozen=True)
+class DockerBottleCleanupPlan(BottleCleanupPlan):
+    """Resources DockerBottleBackend.cleanup will remove. Produced by
+    `prepare_cleanup` from a snapshot of `docker ps -a` + `docker
+    network ls`; sorted so the y/N output is stable."""
+
+    containers: tuple[str, ...]
+    networks: tuple[str, ...]
+
+    @property
+    def empty(self) -> bool:
+        return not self.containers and not self.networks
+
+    def print(self) -> None:
+        print(file=sys.stderr)
+        for name in self.containers:
+            info(f"container: {name}")
+        for name in self.networks:
+            info(f"network:   {name}")
+        print(file=sys.stderr)