feat(backend): remove smolmachines; firecracker is the Linux default

Delete the smolmachines backend (the whole bot_bottle/backend/smolmachines
package and its tests). It had fatal Linux issues (TSI networking under
sustained use, exec-channel contention, no SIGWINCH) and is superseded by
the Firecracker backend (issue #342).

Backend selection now:
- default is macos-container on macOS, firecracker on KVM-capable Linux
  hosts, and docker as the last resort (was smolmachines).
- firecracker is selected on a KVM host even when the `firecracker`
  binary isn't installed, so start routes through its preflight and
  prints an install pointer (same UX as require_container), instead of
  silently falling back. Split is_host_capable() (Linux + KVM) out of
  is_available() (adds the binary check) to drive this.

Retarget the cross-backend tests (parity, print-parity, prepare,
workspace, freezer, selection) from smolmachines to firecracker rather
than dropping the coverage. Remove docker.util.image_id/save, which only
smolmachines used. Update README/AGENTS/example bottles and stale
comments; historical docs/prds are left as a point-in-time record.

BREAKING: BOT_BOTTLE_BACKEND=smolmachines now errors as unknown.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WBMWTEtQdJ4W5UrWuLHCck

bot_bottle/agent_provider.py

@@ -227,9 +227,9 @@ class AgentProvider(ABC):
         from .backend.util import AGENT_CA_PATH, log_ca_fingerprint, select_ca_cert
         from .log import die
         cert_host_path, label = select_ca_cert(plan.egress_plan)
-        # Ensure the target directory exists. smolvm's pack step may not
-        # preserve the empty /usr/local/share/ca-certificates/ directory
-        # on Linux; mkdir -p is idempotent and safe for all backends.
+        # Ensure the target directory exists. A backend's rootfs build
+        # may not preserve the empty /usr/local/share/ca-certificates/
+        # directory; mkdir -p is idempotent and safe for all backends.
         bottle.exec("mkdir -p /usr/local/share/ca-certificates", user="root")
         bottle.cp_in(str(cert_host_path), AGENT_CA_PATH)
         r = bottle.exec(