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sustained use, exec-channel contention, no SIGWINCH) and is superseded by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/backend/firecracker/bottle.py

@@ -4,8 +4,7 @@ Every operation routes through SSH to the guest (dropbear, listening on
 the TAP link). `exec` pipes a script over stdin and captures output;
 `cp_in` uses scp; `exec_agent` uses `ssh -t` for an interactive PTY
 session. `ssh -t` forwards the host terminal's SIGWINCH to the remote
-PTY natively, so — unlike the smolmachines backend — no resize bridge
-is needed.
+PTY natively, so no separate resize bridge is needed.
 
 Commands run as the image's `node` user via `runuser`, with HOME/USER/
 PATH and the bottle env (HTTPS_PROXY at the sidecar, CA paths, …) set