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

README.md

@@ -6,8 +6,8 @@ token from Vault, so automated PRs, comments and pushes are attributed to the
 agent — not to whoever happens to run the command. Set `AGENT_LOGIN` to act as a
 different agent identity.
 
-- **`agentpr`** — create and edit pull requests and issues, and post comments on
-  either, as the agent user.
+- **`agentpr`** — create and edit pull requests and issues, close and reopen
+  issues, and post comments on either, as the agent user.
 - **`watchpr`** — poll one or more PRs and exit when one changes in a way worth
   acting on.
 - **`agentws`** — manage per-branch git worktrees for `unkin-agent`, cloning
@@ -68,6 +68,12 @@ agentpr issue comment --repo unkin/argocd-apps --issue 43 --body "Fixed in #44."
 agentpr issue edit --repo unkin/argocd-apps --issue 43 --body "The pipeline fails with ..."
 # prints: #<number> <html_url>
 
+# Close or reopen an issue; an issue already in that state is an error, not a
+# silent success
+agentpr issue close --repo unkin/argocd-apps --issue 43
+agentpr issue reopen --repo unkin/argocd-apps --issue 43
+# prints: #<number> <state> <html_url>
+
 agentpr --version
 agentpr --help
 ```