feat(bottles): implement bottle factory abstraction per PRD 0003

Introduce claude_bottle/bottles/ with a Bottle Protocol and a
get_bottle_factory() that dispatches on CLAUDE_BOTTLE_PLATFORM
(default "docker"). Move every Docker-specific subprocess.run call
from cli/start.py, plus the orchestration of build, networks, the
pipelock sidecar, container launch, and per-container provisioning
(prompt, skills, ssh, .git), into create_docker_bottle.

Drop bottles[].runtime from the manifest schema. Auto-detect whether
gVisor is registered with the daemon and pass --runtime=runsc when it
is; the preflight shows the resolved runtime so the choice is visible.
Manifests still carrying 'runtime' get a clear error pointing at the
auto-detect behavior, rather than silent ignore.

Out of scope: cli/cleanup.py and cli/list.py still call docker
directly. They enumerate active bottles across the host, which is a
separate concern from "create a bottle" and is left for a follow-up
that introduces a list_active/cleanup primitive on the factory.

claude_bottle/docker.py

@@ -19,22 +19,6 @@ def require_docker() -> None:
         die("docker not found")
 
 
-def require_runsc() -> None:
-    """Fail with an install pointer if the `runsc` (gVisor) runtime is
-    not registered with the local Docker daemon. Called when a bottle
-    sets `runtime: "runsc"`."""
-    result = subprocess.run(
-        ["docker", "info", "--format", "{{json .Runtimes}}"],
-        capture_output=True,
-        text=True,
-    )
-    if result.returncode != 0 or "runsc" not in result.stdout:
-        info("This bottle requested runtime 'runsc' but the gVisor runtime is not registered with Docker.")
-        info("Install gVisor and register it with the daemon:  https://gvisor.dev/docs/user_guide/install/")
-        info("On macOS, gVisor is not available natively; remove 'runtime' from the bottle or run on Linux.")
-        die("runsc runtime not available")
-
-
 def image_exists(ref: str) -> bool:
     return _silent_run(["docker", "image", "inspect", ref]) == 0