feat: add shell completions, command grouping, and dynamic completions

Add completion command for bash/zsh/fish/powershell generation. Organize
help text using Cobra command groups (Task Commands, Reports, Other).
Register dynamic ValidArgsFunction on filter-accepting commands to
suggest +tag and project:name completions from the database.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

opal-task/cmd/completion.go

@@ -0,0 +1,48 @@
+package cmd
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/spf13/cobra"
+)
+
+var completionCmd = &cobra.Command{
+	Use:   "completion [bash|zsh|fish|powershell]",
+	Short: "Generate shell completions",
+	Long: `Generate shell completion scripts for opal.
+
+To load completions:
+
+Bash:
+  $ source <(opal completion bash)
+  # To load on startup, add to ~/.bashrc:
+  $ echo 'source <(opal completion bash)' >> ~/.bashrc
+
+Zsh:
+  $ source <(opal completion zsh)
+  # To load on startup, add to ~/.zshrc:
+  $ echo 'source <(opal completion zsh)' >> ~/.zshrc
+
+Fish:
+  $ opal completion fish | source
+  # To load on startup:
+  $ opal completion fish > ~/.config/fish/completions/opal.fish`,
+	ValidArgs: []string{"bash", "zsh", "fish", "powershell"},
+	Args:      cobra.MatchAll(cobra.ExactArgs(1), cobra.OnlyValidArgs),
+	Run: func(cmd *cobra.Command, args []string) {
+		switch args[0] {
+		case "bash":
+			rootCmd.GenBashCompletion(os.Stdout)
+		case "zsh":
+			rootCmd.GenZshCompletion(os.Stdout)
+		case "fish":
+			rootCmd.GenFishCompletion(os.Stdout, true)
+		case "powershell":
+			rootCmd.GenPowerShellCompletionWithDesc(os.Stdout)
+		default:
+			fmt.Fprintf(os.Stderr, "Unknown shell: %s\n", args[0])
+			os.Exit(1)
+		}
+	},
+}