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/server.go

@@ -198,6 +198,7 @@ Examples:
 }
 
 func init() {
+	serverCmd.GroupID = "other"
 	rootCmd.AddCommand(serverCmd)
 	serverCmd.AddCommand(serverStartCmd)
 	serverCmd.AddCommand(keygenCmd)