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sustained use, exec-channel contention, no SIGWINCH) and is superseded by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

AGENTS.md

@@ -9,11 +9,11 @@ host. A Python CLI (entry point `cli.py`, package `bot_bottle/`) orchestrates
 the runtime lifecycle and the copying of skills and env vars into it.
 The default backend on compatible macOS hosts is macos-container:
 agents and sidecar bundles run through Apple's `container` CLI without
-requiring Docker. The smolmachines backend remains available with
-`BOT_BOTTLE_BACKEND=smolmachines` or `--backend=smolmachines`; agents
-run in a libkrun micro-VM, while the sidecar bundle still uses Docker.
-The legacy Docker backend remains available with `BOT_BOTTLE_BACKEND=docker`
-or `--backend=docker`.
+requiring Docker. On KVM-capable Linux hosts the default is firecracker:
+agents run in a Firecracker microVM reached over SSH on a point-to-point
+TAP, while the sidecar bundle still uses Docker. The legacy Docker
+backend remains available with `BOT_BOTTLE_BACKEND=docker` or
+`--backend=docker`.
 
 ## Goals