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sustained use, exec-channel contention, no SIGWINCH) and is superseded by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/cli/cleanup.py

@@ -1,14 +1,14 @@
 """cleanup: stop and remove all orphaned bot-bottle resources.
 
-Walks every registered backend (docker + smolmachines) so a single
-`./cli.py cleanup` reaps both backends' leftovers — orphaned
-smolvm machines won't survive a docker-only cleanup pass (issue
-addressed alongside #77).
+Walks every registered backend (docker, firecracker, macos-container)
+so a single `./cli.py cleanup` reaps every backend's leftovers — a
+firecracker bottle's sidecars won't survive a docker-only cleanup pass
+(issue addressed alongside #77).
 
 Each backend's `prepare_cleanup` enumerates its own resources;
 docker's `_list_orphan_state_dirs` consults
 `enumerate_active_agents()` for the union of live identities so
-state dirs of running smolmachines bottles aren't reaped. State
+state dirs of running non-docker bottles aren't reaped. State
 dirs are shared layout, so docker is the single owner of that
 bucket.