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/cli/cleanup.py

@@ -5,14 +5,14 @@ from __future__ import annotations
 
 import sys
 
-from ..platform import get_bottle_platform
+from ..backend import get_bottle_backend
 from ..log import info
 from ._common import read_tty_line
 
 
 def cmd_cleanup(_argv: list[str]) -> int:
-    platform = get_bottle_platform()
-    plan = platform.prepare_cleanup()
+    backend = get_bottle_backend()
+    plan = backend.prepare_cleanup()
 
     if plan.empty:
         info("no claude-bottle resources to clean up")
@@ -26,6 +26,6 @@ def cmd_cleanup(_argv: list[str]) -> int:
         info("aborted")
         return 0
 
-    platform.cleanup(plan)
+    backend.cleanup(plan)
     info("done")
     return 0