M6: shell completions with dynamic alias lookup

Add ValidArgsFunction to mount and unmount commands that reads config
aliases for tab completion. Install zsh, bash, and fish completion
scripts via flake postInstall. Update PLAN.md with post-1.0 roadmap.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cmd/mount.go

@@ -15,10 +15,11 @@ import (
 var mountpoint string
 
 var mountCmd = &cobra.Command{
-	Use:   "mount <device|alias>",
-	Short: "Unlock and mount a LUKS volume",
-	Args:  cobra.ExactArgs(1),
-	RunE:  runMount,
+	Use:               "mount <device|alias>",
+	Short:             "Unlock and mount a LUKS volume",
+	Args:              cobra.ExactArgs(1),
+	RunE:              runMount,
+	ValidArgsFunction: completeDeviceOrAlias,
 }
 
 func init() {