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/completions.go

@@ -0,0 +1,50 @@
+package cmd
+
+import (
+	"fmt"
+
+	"git.jnss.me/joakim/opal/internal/engine"
+	"github.com/spf13/cobra"
+)
+
+// taskFilterCompletion provides dynamic completions for task filter arguments.
+// Suggests +tag and project:name completions from the database.
+func taskFilterCompletion(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
+	var completions []string
+
+	tags, err := engine.GetAllTags()
+	if err == nil {
+		for _, tag := range tags {
+			completions = append(completions, fmt.Sprintf("+%s", tag))
+		}
+	}
+
+	projects, err := engine.GetAllProjects()
+	if err == nil {
+		for _, proj := range projects {
+			completions = append(completions, fmt.Sprintf("project:%s", proj))
+		}
+	}
+
+	// Add known attribute keys
+	for key := range engine.ValidAttributeKeys {
+		completions = append(completions, fmt.Sprintf("%s:", key))
+	}
+
+	return completions, cobra.ShellCompDirectiveNoFileComp
+}
+
+func init() {
+	// Register dynamic completions for commands that accept filters
+	addCmd.ValidArgsFunction = taskFilterCompletion
+	doneCmd.ValidArgsFunction = taskFilterCompletion
+	deleteCmd.ValidArgsFunction = taskFilterCompletion
+	modifyCmd.ValidArgsFunction = taskFilterCompletion
+	startCmd.ValidArgsFunction = taskFilterCompletion
+	stopCmd.ValidArgsFunction = taskFilterCompletion
+	editCmd.ValidArgsFunction = taskFilterCompletion
+	infoCmd.ValidArgsFunction = taskFilterCompletion
+	annotateCmd.ValidArgsFunction = taskFilterCompletion
+	denotateCmd.ValidArgsFunction = taskFilterCompletion
+	logCmd.ValidArgsFunction = taskFilterCompletion
+}