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sustained use, exec-channel contention, no SIGWINCH) and is superseded by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

@@ -26,9 +26,9 @@ backend exposes five methods:
 
 Selection is driven by `--backend` on `start` or BOT_BOTTLE_BACKEND
 (env var). When neither is set, compatible macOS hosts default to
-`macos-container`; Linux hosts with Firecracker + KVM default to
-`firecracker`; otherwise `smolmachines`. Per PRD 0003 the manifest
-does not carry a backend field; the host picks.
+`macos-container`; Linux hosts with KVM default to `firecracker`;
+otherwise `docker`. Per PRD 0003 the manifest does not carry a
+backend field; the host picks.
 """
 
 from __future__ import annotations
@@ -98,14 +98,14 @@ class BottlePlan(ABC):
     @property
     def git_gate_insteadof_host(self) -> str:
         """Host (and optional port) used in git-gate insteadOf URLs.
-        Docker uses the compose-network DNS alias; smolmachines
-        overrides with a loopback IP:port since TSI has no DNS."""
+        Docker uses the compose-network DNS alias; VM backends may
+        override with an IP:port when the guest has no DNS."""
         return "git-gate"
 
     @property
     def git_gate_insteadof_scheme(self) -> str:
         """URL scheme for git-gate insteadOf rewrites. 'git' for
-        Docker (git daemon); 'http' for smolmachines (HTTP proxy
+        Docker (git daemon); VM backends may override (e.g. 'http'
         over a published host port)."""
         return "git"
     egress_plan: EgressPlan
@@ -183,8 +183,8 @@ class BottleCleanupPlan(ABC):
 class ExecResult:
     """Captured result of `Bottle.exec`. Backend-neutral: the Docker
     impl populates it from a `subprocess.CompletedProcess`, but a
-    future fly/smolmachines backend could populate it from any source
-    that produces a returncode + captured streams."""
+    VM backend could populate it from any source that produces a
+    returncode + captured streams."""
 
     returncode: int
     stdout: str
@@ -202,7 +202,7 @@ class ActiveAgent:
     of sidecar daemons currently up for this bottle (`egress`,
     `git-gate`, `supervise`); the dashboard uses it to
     gate edit verbs. `backend_name` is the matching key in
-    `_BACKENDS` (`docker` / `smolmachines` / `macos-container`) — used by the active-
+    `_BACKENDS` (`docker` / `firecracker` / `macos-container`) — used by the active-
     list rendering to disambiguate and by the dashboard's
     re-attach path."""
 
@@ -507,7 +507,7 @@ class BottleBackend(ABC, Generic[PlanT, CleanupT]):
         MCP entry inside the guest.
 
         Default returns "" so backends without supervise support
-        don't have to implement it. Docker and smolmachines override."""
+        don't have to implement it. Docker and firecracker override."""
         del plan
         return ""
 
@@ -524,19 +524,19 @@ class BottleBackend(ABC, Generic[PlanT, CleanupT]):
     def enumerate_active(self) -> Sequence[ActiveAgent]:
         """Return every currently-running agent on this backend.
         Empty when none. Backend-specific: docker queries `docker
-        compose ls`; smolmachines queries `smolvm machine ls --json`
-        + cross-references its bundle container."""
+        compose ls`; firecracker cross-references its running sidecar
+        containers against per-bottle metadata."""
 
     @classmethod
     @abstractmethod
     def is_available(cls) -> bool:
         """Whether this backend's runtime prerequisites are satisfied
-        on the current host. Docker → `docker` on PATH; smolmachines
-        → `smolvm` on PATH. Used by the cross-backend
+        on the current host. Docker → `docker` on PATH; firecracker →
+        Linux + KVM. Used by the cross-backend
         `enumerate_active_agents` / `cmd_cleanup` to skip backends
         the operator hasn't installed, so a docker-only host
         doesn't fail when `cli.py list active` walks past
-        smolmachines."""
+        firecracker."""
 
 
 # Import concrete backend classes AFTER the base types are defined, so
@@ -545,7 +545,6 @@ class BottleBackend(ABC, Generic[PlanT, CleanupT]):
 from .docker import DockerBottleBackend  # noqa: E402  # pylint: disable=wrong-import-position
 from .firecracker import FirecrackerBottleBackend  # noqa: E402  # pylint: disable=wrong-import-position
 from .macos_container import MacosContainerBottleBackend  # noqa: E402  # pylint: disable=wrong-import-position
-from .smolmachines import SmolmachinesBottleBackend  # noqa: E402  # pylint: disable=wrong-import-position
 
 # Freezer is imported after the backend classes for the same reason:
 # Freezer.commit_slug constructs ActiveAgent, which must be fully
@@ -561,7 +560,6 @@ _BACKENDS: dict[str, BottleBackend[Any, Any]] = {
     "docker": DockerBottleBackend(),
     "firecracker": FirecrackerBottleBackend(),
     "macos-container": MacosContainerBottleBackend(),
-    "smolmachines": SmolmachinesBottleBackend(),
 }
 
 
@@ -574,7 +572,8 @@ def get_bottle_backend(
       1. explicit arg (CLI `--backend=<name>` passes through here)
       2. BOT_BOTTLE_BACKEND env var
       3. `macos-container` on compatible macOS hosts
-      4. default `smolmachines`
+      4. `firecracker` on KVM-capable Linux hosts
+      5. default `docker`
 
     Dies with a pointer at the known backends if the chosen name
     isn't implemented."""
@@ -588,9 +587,13 @@ def get_bottle_backend(
 def _default_backend_name() -> str:
     if has_backend("macos-container"):
         return "macos-container"
-    if has_backend("firecracker"):
+    # A KVM-capable Linux host defaults to firecracker even when the
+    # `firecracker` binary isn't installed yet: selecting it here routes
+    # start through firecracker's preflight, which prints an install
+    # pointer, instead of silently falling back to docker.
+    if FirecrackerBottleBackend.is_host_capable():
         return "firecracker"
-    return "smolmachines"
+    return "docker"
 
 
 def known_backend_names() -> tuple[str, ...]:
@@ -604,7 +607,7 @@ def has_backend(name: str) -> bool:
     """Whether the named backend's runtime prerequisites are
     available on the current host. Cross-backend callers (list,
     cleanup) skip unavailable backends so a docker-only host
-    doesn't fail when the smolmachines backend isn't installed,
+    doesn't fail when the firecracker backend isn't usable,
     and vice versa.
 
     Returns False for unknown names so callers can pass