agentpr: close and reopen issues

- add issue close and issue reopen subcommands
- add GetIssue and SetIssueState to the Gitea client
- fail when the issue is already in the requested state
- document the new subcommands in README.md and AGENTS.md

internal/agent/gitea.go

@@ -329,6 +329,46 @@ func (c *GiteaClient) EditIssue(repoPath string, number int, opts EditOptions) (
 	return issue, err
 }
 
+// Issue states Gitea accepts on a state change. Gitea has no third state: an
+// issue is open or closed.
+const (
+	IssueStateOpen   = "open"
+	IssueStateClosed = "closed"
+)
+
+// ErrIssueStateUnchanged reports a state change asked for the state the issue
+// is already in. Gitea answers such a PATCH with 200 and changes nothing, so
+// without this check closing an already-closed issue would look like it worked.
+var ErrIssueStateUnchanged = errors.New("issue is already in that state")
+
+// GetIssue fetches a single issue
+// (GET /api/v1/repos/{owner}/{repo}/issues/{index}).
+func (c *GiteaClient) GetIssue(repoPath string, number int) (Issue, error) {
+	var issue Issue
+	err := c.do(http.MethodGet, fmt.Sprintf("/api/v1/repos/%s/issues/%d", repoPath, number), nil, &issue)
+	return issue, err
+}
+
+// SetIssueState closes or reopens an issue. It reads the issue first so an
+// issue already in the requested state fails with ErrIssueStateUnchanged
+// instead of reporting a change that never happened.
+func (c *GiteaClient) SetIssueState(repoPath string, number int, state string) (Issue, error) {
+	if state != IssueStateOpen && state != IssueStateClosed {
+		return Issue{}, fmt.Errorf("invalid issue state %q: want %q or %q", state, IssueStateOpen, IssueStateClosed)
+	}
+	current, err := c.GetIssue(repoPath, number)
+	if err != nil {
+		return Issue{}, err
+	}
+	if current.State == state {
+		return current, fmt.Errorf("%s#%d: %w (%s)", repoPath, number, ErrIssueStateUnchanged, state)
+	}
+	var issue Issue
+	payload := map[string]string{"state": state}
+	err = c.do(http.MethodPatch, fmt.Sprintf("/api/v1/repos/%s/issues/%d", repoPath, number), payload, &issue)
+	return issue, err
+}
+
 // Comment is the subset of an issue comment we track.
 type Comment struct {
 	ID   int64  `json:"id"`