feat(bottle): opt-in gVisor runtime per bottle

Bottles can now set "runtime": "runsc" to launch the agent container
under gVisor instead of runc, adding a userspace syscall barrier
between the agent and the host kernel. Default is runc (Docker
default). Pipelock stays on the default runtime per the research doc's
minimum-diff prescription.

The launcher verifies runsc is registered with the daemon before
launch, surfaces the runtime in the preflight plan, and dies with an
install pointer (and a macOS-not-supported note) when runsc is
requested but unavailable.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

claude-bottle.example.json

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