feat(issue): add get_discussion_markdown to issue_read

Adds a new issue_read method that fetches an issue and its comments
and renders them as a single Markdown document, instead of a JSON
array. Formatting logic lives in a pure, unit-tested helper
(formatDiscussionMarkdown) covering comment rendering, empty comment
lists, and attachment-inlined bodies. Existing get/get_comments/
get_labels methods are unchanged.

Co-Authored-By: Codet <codet@commitgo.dev> (GPT-5-Codex)

operation/issue/markdown.go

@@ -0,0 +1,63 @@
+package issue
+
+import (
+	"fmt"
+	"strings"
+	"time"
+)
+
+type discussionIssue struct {
+	Number    int64
+	Title     string
+	Author    string
+	State     string
+	Labels    []string
+	Body      string
+	CreatedAt time.Time
+}
+
+type discussionComment struct {
+	Author    string
+	CreatedAt time.Time
+	Body      string
+}
+
+// formatDiscussionMarkdown renders an issue and its comments as a single
+// Markdown document suitable for display without further parsing.
+func formatDiscussionMarkdown(issue discussionIssue, comments []discussionComment) string {
+	var b strings.Builder
+
+	fmt.Fprintf(&b, "# %s (#%d)\n\n", issue.Title, issue.Number)
+	fmt.Fprintf(&b, "**Author:** %s\n", issue.Author)
+	fmt.Fprintf(&b, "**State:** %s\n", issue.State)
+	if len(issue.Labels) > 0 {
+		fmt.Fprintf(&b, "**Labels:** %s\n", strings.Join(issue.Labels, ", "))
+	}
+	if !issue.CreatedAt.IsZero() {
+		fmt.Fprintf(&b, "**Created:** %s\n", issue.CreatedAt.UTC().Format(time.RFC3339))
+	}
+	b.WriteString("\n")
+	b.WriteString(strings.TrimSpace(issue.Body))
+	b.WriteString("\n\n---\n\n## Comments\n\n")
+
+	if len(comments) == 0 {
+		b.WriteString("_No comments yet._\n")
+		return b.String()
+	}
+
+	for i, c := range comments {
+		fmt.Fprintf(&b, "### %s", c.Author)
+		if !c.CreatedAt.IsZero() {
+			fmt.Fprintf(&b, " on %s", c.CreatedAt.UTC().Format(time.RFC3339))
+		}
+		b.WriteString("\n\n")
+		b.WriteString(strings.TrimSpace(c.Body))
+		if i < len(comments)-1 {
+			b.WriteString("\n\n---\n\n")
+		} else {
+			b.WriteString("\n")
+		}
+	}
+
+	return b.String()
+}