feat(sbx,ai): snapshot all backends w/ local export; robust AI + rejoin

- /sbx save [--local] and new /sbx vmsave/vmsnaps: snapshot Docker,
  Multipass, and VirtualBox; --local also writes a portable artifact
  (docker .tar / VBox .ova) under hh-snapshots/ that survives pruning.
- sandbox reappears for anyone who leaves and rejoins (host replays
  status + screen snapshot + ACL on Joined); SbxStatus ready handler is
  now idempotent so it never wipes scrollback.
- received files auto-bridge into the hosted sandbox (ft::tar_path).
- AI agent: translate Ollama's cryptic 404 into model/host/fix guidance.
- bootstrap installs the AI layer (Ollama + default model) by default,
  with consent gates; --no-ai opts out, --yes skips prompts.
- help menu lists the new save/vm flags.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

hh/src/ui.rs

@@ -155,8 +155,8 @@ fn help_clusters(theme: &Theme) -> Vec<HelpCluster> {
                 ),
                 kv("/sbx stop", "tear down the sandbox (purges the VM)"),
                 kv(
-                    "/sbx save [label]",
-                    "snapshot state (docker image; survives stop)",
+                    "/sbx save [label] [--local]",
+                    "snapshot state (docker image; --local also writes a portable .tar)",
                 ),
                 kv(
                     "/sbx load <label>",
@@ -185,6 +185,11 @@ fn help_clusters(theme: &Theme) -> Vec<HelpCluster> {
                     "/sbx gui cancel",
                     "abort a pending launch (nothing stopped or installed)",
                 ),
+                kv(
+                    "/sbx vmsave <vm> [label] [--local]",
+                    "snapshot a VM (--local also exports a portable .ova)",
+                ),
+                kv("/sbx vmsnaps <vm>", "list a VM's snapshots"),
             ],
         },
         HelpCluster {