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.example.json

@@ -13,7 +13,6 @@
     },
 
     "gitea-dev": {
-      "runtime": "runsc",
       "env": {
         "GITEA_TOKEN":     "?paste your Gitea API token",
         "GITHUB_TOKEN":    "${GH_PAT}",