feat(exec): allow cocoon vm exec on Windows guests

cocoon-agent v0.1.2 ships with Windows SCM service mode + AF_VSOCK +
viosock support, so the historical "not yet available on Windows"
guard no longer reflects reality. Drop it; the same `dialHybridVsock`
+ `client.Run` flow already handles Windows guests with no further
changes.

Validated on `35.240.182.52` against a 4-deep clone chain:

  cocoon vm exec win-c{1,2,3} -- powershell -NoProfile -Command ...

works on every clone — the vsock path stays up even while the NIC is
mid-hot-rebind on `--on-demand` clone3 (issue#28 NDIS-stuck symptom),
which makes vsock exec the only host-side control channel that
survives clone resume independent of guest networking.

Author: CMGS

cmd/vm/exec.go

@@ -53,9 +53,6 @@ func (h Handler) Exec(cmd *cobra.Command, args []string) error {
 	if info.VsockSocket == "" {
 		return fmt.Errorf("exec: %w (recreate the VM to enable agent exec)", ErrVsockNotConfigured)
 	}
-	if info.Config.Windows {
-		return fmt.Errorf("exec: cocoon-agent is not yet available on Windows guests")
-	}
 
 	envPairs, _ := cmd.Flags().GetStringArray("env")
 	env, err := parseExecEnv(envPairs)