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/completion.go

@@ -0,0 +1,25 @@
+package cmd
+
+import (
+	"sort"
+
+	"git.wntrmute.dev/kyle/arca/internal/config"
+	"github.com/spf13/cobra"
+)
+
+// completeDeviceOrAlias provides dynamic completion for device aliases
+// and device paths from the config file.
+func completeDeviceOrAlias(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
+	if len(args) > 0 {
+		return nil, cobra.ShellCompDirectiveNoFileComp
+	}
+
+	cfg := config.Load()
+	var completions []string
+	for alias := range cfg.Devices {
+		completions = append(completions, alias)
+	}
+	sort.Strings(completions)
+
+	return completions, cobra.ShellCompDirectiveNoFileComp
+}