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sustained use, exec-channel contention, no SIGWINCH) and is superseded by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

@@ -76,14 +76,14 @@ def git_gate_render_gitconfig(
     entries: tuple[ManifestGitEntry, ...], gate_host: str, *, scheme: str = "git",
 ) -> str:
     """Render the agent's ~/.gitconfig content for git-gate
-    `insteadOf` rewrites. Pure host-side, no docker / smolvm;
+    `insteadOf` rewrites. Pure host-side, no docker / VM;
     exposed for tests + reuse across backends.
 
     `gate_host` is the part of the URL between `<scheme>://` and the
     repo path — backends differ here:
       - docker:        `git-gate` (the short network alias)
-      - smolmachines:  `<bundle_ip>:<port>` (no DNS in the
-                       TSI-allowlisted guest)
+      - firecracker:   `<bundle_ip>:<port>` (no DNS on the
+                       point-to-point TAP link)
 
     Empty `entries` returns an empty string so callers can no-op
     cleanly without conditional formatting at the call site."""