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/util.py

@@ -268,6 +268,10 @@ def ssh_base_argv(private_key: Path, guest_ip: str) -> list[str]:
     return [
         "ssh",
         "-i", str(private_key),
+        # Only offer the per-bottle key — don't let an agent or the
+        # operator's ~/.ssh/config inject other identities (newer
+        # OpenSSH otherwise may not reliably present the -i key).
+        "-o", "IdentitiesOnly=yes",
         "-o", "StrictHostKeyChecking=no",
         "-o", "UserKnownHostsFile=/dev/null",
         "-o", "LogLevel=ERROR",
@@ -301,9 +305,17 @@ if [ -n "$KEY" ]; then
     chown -R node:node /home/node/.ssh 2>/dev/null || true
 fi
 
+# The rootless rootfs build (`docker export | tar` as a non-root user)
+# can't preserve uid 0, so every path lands owned by the build uid,
+# which maps to `node` (uid 1000) in-guest — including /root. dropbear
+# (like OpenSSH) refuses root's authorized_keys unless the home dir is
+# owned by root, so restore root's ownership of its own home.
+chown -R 0:0 /root 2>/dev/null || true
+
 mkdir -p /etc/dropbear /run
-# -R: generate host keys on demand. -E: log to stderr (guest console).
-/bb-dropbear -R -E -p 22 2>/dev/null &
+# -R: generate host keys on demand. -E: log auth failures to stderr,
+# captured in the host-side console.log for debugging.
+/bb-dropbear -R -E -p 22 &
 
 # Reap zombies as PID 1. dropbear is always a child, so `wait` blocks
 # rather than busy-looping.