fix(firecracker): make the launch path work end-to-end

First real end-to-end launch (there was no firecracker integration test)
surfaced three bugs in the control/provision path, all now verified fixed
by tests/integration/test_firecracker_launch:

1. Guest SSH rejected the correct key. The rootless rootfs build
   (`docker export | tar` as a non-root user) can't preserve uid 0, so
   every path — including /root — is owned by the build uid (node in
   guest). dropbear refuses root's authorized_keys when /root isn't
   root-owned, so auth fell back to password → denied. bb-init now
   `chown -R 0:0 /root`.

2. The SSH client (newer OpenSSH) didn't reliably present the -i key
   against the operator's ~/.ssh config; add `IdentitiesOnly=yes` to
   ssh_base_argv so only the per-bottle key is offered.

3. cp_in double-wrapped the remote command as `sh -c <remote>`, but ssh
   space-joins everything after the host into one string for the guest
   shell, collapsing `sh -c mkdir -p X && …` to `mkdir` with no operand
   ("missing operand"). Pass the command as a single arg and let the
   guest login shell run it (stdin still carries the tar).

Also stop discarding dropbear's stderr (`-E` now reaches the host-side
console.log) so future guest-auth issues are debuggable.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WBMWTEtQdJ4W5UrWuLHCck

bot_bottle/backend/firecracker/bottle.py

@@ -151,8 +151,13 @@ class FirecrackerBottle(Bottle):
             ["tar", "-C", host_parent, "-cf", "-", host_base],
             stdout=subprocess.PIPE,
         )
+        # Pass the command as one arg: ssh space-joins everything after
+        # the host into a single string for the guest's login shell, so a
+        # `sh -c <remote>` split would drop everything past the first word
+        # (the guest shell runs `<remote>` directly; stdin carries the
+        # tar).
         ssh = subprocess.run(
-            [*self._ssh(tty=False), "--", "sh", "-c", remote],
+            [*self._ssh(tty=False), "--", remote],
             stdin=tar.stdout, capture_output=True, text=True, check=False,
         )
         tar.wait()