feat: add Vikunja issues provider and integrate tasks retrieval

Co-authored-by: aider (openai/qwen2.5-coder:32b-instruct-q4_0) <aider@aider.chat>

issues/vikunja/vikunja.go

@@ -0,0 +1,47 @@
+package vikunja
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"os"
+)
+
+type Task struct {
+	Title       string `json:"title"`
+	Description string `json:"description"`
+	Status      int    `json:"status"` // Assuming 1 is for completed tasks
+}
+
+func GetTasks(instance, user, from, to string) ([]Task, error) {
+	req, err := http.NewRequest(http.MethodGet, fmt.Sprintf("https://%s/api/v1/user/tasks", instance), nil)
+	if err != nil {
+		return nil, fmt.Errorf("error building vikunja request: %w", err)
+	}
+	q := req.URL.Query()
+	q.Add("user_id", user)
+	q.Add("status", "1") // Assuming 1 is for completed tasks
+	req.URL.RawQuery = q.Encode()
+	req.Header.Add("Authorization", fmt.Sprintf("Bearer %s", os.Getenv("VIKUNJA_TOKEN")))
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("error executing vikunja request: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode >= 400 {
+		b := bytes.NewBuffer(nil)
+		io.Copy(b, resp.Body)
+		return nil, fmt.Errorf("error talking to vikunja: %s", b.String())
+	}
+
+	var tasks []Task
+	err = json.NewDecoder(resp.Body).Decode(&tasks)
+	if err != nil {
+		return nil, fmt.Errorf("error decoding vikunja response: %w", err)
+	}
+
+	return tasks, nil
+}