Add notification_read and notification_write tools (#172)

Add notification management via two new MCP tools using the method-dispatch pattern:

- `notification_read`: list notifications (global or repo-scoped, with status/subject_type/since/before filters) and get single notification thread by ID
- `notification_write`: mark single notification as read, mark all notifications as read (global or repo-scoped)

---
This PR was written with the help of Claude Opus 4.6

Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/172
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-committed-by: silverwind <me@silverwind.io>

operation/notification/slim.go

@@ -0,0 +1,66 @@
+package notification
+
+import (
+	gitea_sdk "code.gitea.io/sdk/gitea"
+)
+
+func slimThread(t *gitea_sdk.NotificationThread) map[string]any {
+	if t == nil {
+		return nil
+	}
+	m := map[string]any{
+		"id":         t.ID,
+		"unread":     t.Unread,
+		"updated_at": t.UpdatedAt,
+	}
+	if t.Pinned {
+		m["pinned"] = true
+	}
+	if t.Repository != nil {
+		m["repository"] = t.Repository.FullName
+	}
+	if t.Subject != nil {
+		subject := map[string]any{
+			"title": t.Subject.Title,
+			"type":  t.Subject.Type,
+			"state": t.Subject.State,
+		}
+		if t.Subject.HTMLURL != "" {
+			subject["html_url"] = t.Subject.HTMLURL
+		}
+		if t.Subject.LatestCommentHTMLURL != "" {
+			subject["latest_comment_html_url"] = t.Subject.LatestCommentHTMLURL
+		}
+		m["subject"] = subject
+	}
+	return m
+}
+
+func slimThreads(threads []*gitea_sdk.NotificationThread) []map[string]any {
+	out := make([]map[string]any, 0, len(threads))
+	for _, t := range threads {
+		if t == nil {
+			continue
+		}
+		m := map[string]any{
+			"id":         t.ID,
+			"unread":     t.Unread,
+			"updated_at": t.UpdatedAt,
+		}
+		if t.Pinned {
+			m["pinned"] = true
+		}
+		if t.Repository != nil {
+			m["repository"] = t.Repository.FullName
+		}
+		if t.Subject != nil {
+			m["subject"] = map[string]any{
+				"title": t.Subject.Title,
+				"type":  t.Subject.Type,
+				"state": t.Subject.State,
+			}
+		}
+		out = append(out, m)
+	}
+	return out
+}