v0.107.0: key-auth-first bridge + staged-secret wipe on escrow confirm

Key-auth-first: a KeyAuthProber seam lets the bridge skip consume+install
when the already-installed key still authenticates (pinned to the freshly
verified host key) — descriptor changes on provisioned guests no longer
loop on consume-404. Fingerprint verify still precedes everything.

Wipe-on-escrowed: confirm-escrow now calls the agent's new
DELETE /escrow/stage-secret (v0.78.0) best-effort, closing the hygiene gap
where a ceremony-less confirm left the staged password file behind.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PSK5g6qYLknKj8u3QAFEr6

controller/cmd/controller/main.go

@@ -242,6 +242,7 @@ func main() {
 			Scanner:   offsiteapply.KeyscanScanner{},
 			KeyGen:    offsiteapply.ED25519KeyGen{},
 			Installer: offsiteapply.SSHCopyIDInstaller{},
+			Prober:    offsiteapply.SFTPKeyAuthProber{KeyPath: filepath.Join(cfg.Paths.DataDir, "offbox", "ssh_key")},
 			Enabler: offsiteapply.EnablerFunc(func(ctx context.Context, host, user string, port int, repoPath, priv, kh string) error {
 				tgt := &settings.OffboxTarget{Enabled: true, Host: host, User: user, Port: port, RepoPath: repoPath, Schedule: "daily"}
 				stage := func(ctx context.Context, pw string) error {