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/root.go

@@ -34,7 +34,7 @@ var commandNames = []string{
 	"add", "done", "modify", "delete",
 	"start", "stop", "count", "projects", "tags",
 	"info", "edit", "server", "sync", "reports", "setup",
-	"version", "annotate", "denotate", "undo", "log",
+	"version", "annotate", "denotate", "undo", "log", "completion",
 }
 
 // Report names (dynamically populated)
@@ -222,31 +222,62 @@ func init() {
 		initializeApp()
 	}
 
-	// Add regular subcommands
+	// Command groups for organized help output
+	rootCmd.AddGroup(
+		&cobra.Group{ID: "task", Title: "Task Commands:"},
+		&cobra.Group{ID: "report", Title: "Reports:"},
+		&cobra.Group{ID: "other", Title: "Other:"},
+	)
+
+	// Task commands
+	addCmd.GroupID = "task"
+	doneCmd.GroupID = "task"
+	modifyCmd.GroupID = "task"
+	deleteCmd.GroupID = "task"
+	startCmd.GroupID = "task"
+	stopCmd.GroupID = "task"
+	editCmd.GroupID = "task"
+	infoCmd.GroupID = "task"
+	annotateCmd.GroupID = "task"
+	denotateCmd.GroupID = "task"
+	undoCmd.GroupID = "task"
+	logCmd.GroupID = "task"
+
 	rootCmd.AddCommand(addCmd)
 	rootCmd.AddCommand(doneCmd)
 	rootCmd.AddCommand(modifyCmd)
 	rootCmd.AddCommand(deleteCmd)
 	rootCmd.AddCommand(startCmd)
 	rootCmd.AddCommand(stopCmd)
-	rootCmd.AddCommand(countCmd)
-	rootCmd.AddCommand(projectsCmd)
-	rootCmd.AddCommand(tagsCmd)
 	rootCmd.AddCommand(infoCmd)
 	rootCmd.AddCommand(editCmd)
-	rootCmd.AddCommand(reportsCmd)
-	rootCmd.AddCommand(versionCmd)
 	rootCmd.AddCommand(annotateCmd)
 	rootCmd.AddCommand(denotateCmd)
 	rootCmd.AddCommand(undoCmd)
 	rootCmd.AddCommand(logCmd)
 
+	// Other commands
+	countCmd.GroupID = "other"
+	projectsCmd.GroupID = "other"
+	tagsCmd.GroupID = "other"
+	reportsCmd.GroupID = "other"
+	versionCmd.GroupID = "other"
+	completionCmd.GroupID = "other"
+
+	rootCmd.AddCommand(countCmd)
+	rootCmd.AddCommand(projectsCmd)
+	rootCmd.AddCommand(tagsCmd)
+	rootCmd.AddCommand(reportsCmd)
+	rootCmd.AddCommand(versionCmd)
+	rootCmd.AddCommand(completionCmd)
+
 	// Enable --version flag on root command
 	rootCmd.Version = Version
 
 	// Add report commands dynamically
 	reportCommands := CreateReportCommands()
 	for _, cmd := range reportCommands {
+		cmd.GroupID = "report"
 		rootCmd.AddCommand(cmd)
 	}
 }