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.py

@@ -0,0 +1,46 @@
+"""DockerBottle — concrete Bottle handle yielded by
+DockerBottleBackend.launch.
+
+Holds the container name plus the in-container prompt path so
+exec_claude can transparently add --append-system-prompt-file when a
+prompt was provisioned.
+"""
+
+from __future__ import annotations
+
+import subprocess
+
+from .. import Bottle
+
+
+class DockerBottle(Bottle):
+    """Concrete Bottle for Docker."""
+
+    def __init__(self, container: str, teardown, prompt_path_in_container: str | None):
+        self.name = container
+        self._teardown = teardown
+        self._prompt_path = prompt_path_in_container
+        self._closed = False
+
+    def exec_claude(self, argv: list[str], *, tty: bool = True) -> int:
+        full_argv = list(argv)
+        if self._prompt_path:
+            full_argv.extend(["--append-system-prompt-file", self._prompt_path])
+        cmd = ["docker", "exec"]
+        if tty:
+            cmd.append("-it")
+        cmd.extend([self.name, "claude", *full_argv])
+        return subprocess.run(cmd).returncode
+
+    def cp_in(self, host_path: str, container_path: str) -> None:
+        subprocess.run(
+            ["docker", "cp", host_path, f"{self.name}:{container_path}"],
+            stdout=subprocess.DEVNULL,
+            check=True,
+        )
+
+    def close(self) -> None:
+        if self._closed:
+            return
+        self._closed = True
+        self._teardown()