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/__init__.py

@@ -0,0 +1,29 @@
+"""Docker bottle backend.
+
+The bulk of the implementation lives in sibling modules:
+
+  - util:                 thin Docker subprocess wrappers
+  - network:              Docker network plumbing
+  - bottle_plan:          DockerBottlePlan
+  - bottle_cleanup_plan:  DockerBottleCleanupPlan
+  - bottle:               DockerBottle handle
+  - backend:              DockerBottleBackend
+
+This file only re-exports the public names so
+`from claude_bottle.backend.docker import DockerBottleBackend` keeps
+working.
+"""
+
+from __future__ import annotations
+
+from .backend import DockerBottleBackend
+from .bottle import DockerBottle
+from .bottle_cleanup_plan import DockerBottleCleanupPlan
+from .bottle_plan import DockerBottlePlan
+
+__all__ = [
+    "DockerBottle",
+    "DockerBottleBackend",
+    "DockerBottleCleanupPlan",
+    "DockerBottlePlan",
+]