feat: implement CloseIssue and PostComment methods in gitea client

Add CloseIssue (PATCH state=closed) and PostComment (POST comment body)
methods to the Gitea client with cache invalidation. Add corresponding
handler routes POST /issues/{owner}/{repo}/{index}/close and
POST /issues/{owner}/{repo}/{index}/comment with HTMX support.
Include unit tests for both client methods.

Closes leeworks-agents/gitea-mobile#36

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/gitea/client.go

@@ -706,6 +706,51 @@ func (c *Client) SubmitReview(ctx context.Context, token, owner, repo string, in
 	return nil
 }
 
+// CloseIssue closes an issue by setting its state to "closed".
+func (c *Client) CloseIssue(ctx context.Context, token, owner, repo string, index int64) error {
+	payload, err := json.Marshal(map[string]string{"state": "closed"})
+	if err != nil {
+		return fmt.Errorf("marshaling close request: %w", err)
+	}
+
+	path := fmt.Sprintf("/repos/%s/%s/issues/%d", owner, repo, index)
+	resp, err := c.doRequest(ctx, token, http.MethodPatch, path, strings.NewReader(string(payload)))
+	if err != nil {
+		return fmt.Errorf("closing issue: %w", err)
+	}
+	resp.Body.Close()
+
+	c.InvalidateAll()
+	return nil
+}
+
+// PostComment creates a comment on an issue and returns the created Comment.
+func (c *Client) PostComment(ctx context.Context, token, owner, repo string, index int64, body string) (*Comment, error) {
+	payload, err := json.Marshal(map[string]string{"body": body})
+	if err != nil {
+		return nil, fmt.Errorf("marshaling comment: %w", err)
+	}
+
+	path := fmt.Sprintf("/repos/%s/%s/issues/%d/comments", owner, repo, index)
+	resp, err := c.doRequest(ctx, token, http.MethodPost, path, strings.NewReader(string(payload)))
+	if err != nil {
+		return nil, fmt.Errorf("posting comment: %w", err)
+	}
+	defer resp.Body.Close()
+
+	var comment Comment
+	if err := json.NewDecoder(resp.Body).Decode(&comment); err != nil {
+		return nil, fmt.Errorf("decoding comment: %w", err)
+	}
+
+	// Populate convenience fields.
+	comment.User = comment.RawUser.Login
+	comment.CreatedAt = comment.RawCreatedAt.Format("Jan 2, 2006 15:04")
+
+	c.InvalidateAll()
+	return &comment, nil
+}
+
 // priorityScore returns a numeric score for sorting (lower = higher priority).
 func priorityScore(labels []string) int {
 	for _, l := range labels {