list: add --format flag with template support

Add flexible --format flag to 'bd list' command supporting:
- Built-in presets: 'digraph' (basic 'from to' format) and 'dot' (Graphviz)
- Custom Go templates for dependency output
- Template variables: IssueID, DependsOnID, Type, Issue, Dependency

This enables graph analysis with tools like golang.org/x/tools/cmd/digraph while allowing users to customize output format for their specific needs.

Examples:
  bd list --format=digraph | digraph nodes
  bd list --format=dot | dot -Tsvg -o deps.svg
  bd list --format='{{.IssueID}} -> {{.DependsOnID}} [{{.Type}}]'

cmd/bd/list.go

@@ -1,11 +1,14 @@
 package main
 
 import (
+	"bytes"
 	"context"
 	"fmt"
 	"os"
+	"text/template"
 
 	"github.com/spf13/cobra"
+	"github.com/steveyegge/beads/internal/storage"
 	"github.com/steveyegge/beads/internal/types"
 )
 
@@ -17,6 +20,7 @@ var listCmd = &cobra.Command{
 		assignee, _ := cmd.Flags().GetString("assignee")
 		issueType, _ := cmd.Flags().GetString("type")
 		limit, _ := cmd.Flags().GetInt("limit")
+		formatStr, _ := cmd.Flags().GetString("format")
 
 		filter := types.IssueFilter{
 			Limit: limit,
@@ -45,6 +49,15 @@ var listCmd = &cobra.Command{
 			os.Exit(1)
 		}
 
+		// Handle format flag
+		if formatStr != "" {
+			if err := outputFormattedList(ctx, store, issues, formatStr); err != nil {
+				fmt.Fprintf(os.Stderr, "Error: %v\n", err)
+				os.Exit(1)
+			}
+			return
+		}
+
 		if jsonOutput {
 			outputJSON(issues)
 			return
@@ -68,5 +81,123 @@ func init() {
 	listCmd.Flags().StringP("assignee", "a", "", "Filter by assignee")
 	listCmd.Flags().StringP("type", "t", "", "Filter by type (bug, feature, task, epic, chore)")
 	listCmd.Flags().IntP("limit", "n", 0, "Limit results")
+	listCmd.Flags().String("format", "", "Output format: 'digraph' (for golang.org/x/tools/cmd/digraph), 'dot' (Graphviz), or Go template")
 	rootCmd.AddCommand(listCmd)
 }
+
+// outputDotFormat outputs issues in Graphviz DOT format
+func outputDotFormat(ctx context.Context, store storage.Storage, issues []*types.Issue) error {
+	fmt.Println("digraph dependencies {")
+	fmt.Println("  rankdir=TB;")
+	fmt.Println("  node [shape=box, style=rounded];")
+	fmt.Println()
+
+	// Build map of all issues for quick lookup
+	issueMap := make(map[string]*types.Issue)
+	for _, issue := range issues {
+		issueMap[issue.ID] = issue
+	}
+
+	// Output nodes with labels
+	for _, issue := range issues {
+		// Escape quotes in title
+		title := issue.Title
+		title = fmt.Sprintf("%q", title) // Go's %q handles escaping
+		fmt.Printf("  %q [label=%s];\n", issue.ID, title)
+	}
+	fmt.Println()
+
+	// Output edges with labels for dependency type
+	for _, issue := range issues {
+		deps, err := store.GetDependencyRecords(ctx, issue.ID)
+		if err != nil {
+			continue
+		}
+		for _, dep := range deps {
+			// Only output edges where both nodes are in the filtered list
+			if issueMap[dep.DependsOnID] != nil {
+				// Color code by dependency type
+				color := "black"
+				style := "solid"
+				switch dep.Type {
+				case "blocks":
+					color = "red"
+					style = "bold"
+				case "parent-child":
+					color = "blue"
+				case "discovered-from":
+					color = "green"
+					style = "dashed"
+				case "related":
+					color = "gray"
+					style = "dashed"
+				}
+				fmt.Printf("  %q -> %q [label=%q, color=%s, style=%s];\n",
+					issue.ID, dep.DependsOnID, dep.Type, color, style)
+			}
+		}
+	}
+
+	fmt.Println("}")
+	return nil
+}
+
+// outputFormattedList outputs issues in a custom format (preset or Go template)
+func outputFormattedList(ctx context.Context, store storage.Storage, issues []*types.Issue, formatStr string) error {
+	// Handle special 'dot' format (Graphviz output)
+	if formatStr == "dot" {
+		return outputDotFormat(ctx, store, issues)
+	}
+
+	// Built-in format presets
+	presets := map[string]string{
+		"digraph": "{{.IssueID}} {{.DependsOnID}}",
+	}
+
+	// Check if it's a preset
+	templateStr, isPreset := presets[formatStr]
+	if !isPreset {
+		templateStr = formatStr
+	}
+
+	// Parse template
+	tmpl, err := template.New("format").Parse(templateStr)
+	if err != nil {
+		return fmt.Errorf("invalid format template: %w", err)
+	}
+
+	// Build map of all issues for quick lookup
+	issueMap := make(map[string]bool)
+	for _, issue := range issues {
+		issueMap[issue.ID] = true
+	}
+
+	// For each issue, output its dependencies using the template
+	for _, issue := range issues {
+		deps, err := store.GetDependencyRecords(ctx, issue.ID)
+		if err != nil {
+			continue
+		}
+		for _, dep := range deps {
+			// Only output edges where both nodes are in the filtered list
+			if issueMap[dep.DependsOnID] {
+				// Template data includes both issue and dependency info
+				data := map[string]interface{}{
+					"IssueID":     issue.ID,
+					"DependsOnID": dep.DependsOnID,
+					"Type":        dep.Type,
+					"Issue":       issue,
+					"Dependency":  dep,
+				}
+
+				var buf bytes.Buffer
+				if err := tmpl.Execute(&buf, data); err != nil {
+					return fmt.Errorf("template execution error: %w", err)
+				}
+				fmt.Println(buf.String())
+			}
+		}
+	}
+
+	return nil
+}