Add agentpr issue create/comment/edit

Filing an issue as unkin-agent means hand-rolling a POST to the issues
API with a manually minted token, the exact credential handling agentpr
exists to encapsulate.

- add `issue create`, `issue comment` and `issue edit`, mirroring the pr
  command group's flags, output and errors
- add CreateIssue/EditIssue to the Gitea client, sharing EditOptions with
  EditPR
- build both comment commands from one constructor so pr and issue
  comments cannot drift off Gitea's shared endpoint
- cover create, edit, a missing repo, an API error and missing flags

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 post PR comments as the
-  agent user.
+- **`agentpr`** — create and edit pull requests and 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
@@ -56,6 +56,18 @@ agentpr pr edit --repo unkin/argocd-apps --pr 42 --body "Adds the ServiceAccount
 agentpr pr edit --repo unkin/argocd-apps --pr 42 --title "Add woodpecker SA"
 # prints: #<number> <html_url>
 
+# File an issue (--body optional)
+agentpr issue create --repo unkin/argocd-apps \
+  --title "Woodpecker SA missing" --body "The pipeline fails with ..."
+# prints: #<number> <html_url>
+
+# Comment on an issue (the same Gitea endpoint `pr comment` posts to)
+agentpr issue comment --repo unkin/argocd-apps --issue 43 --body "Fixed in #44."
+
+# Edit an issue's title and/or body; an omitted flag is left unchanged
+agentpr issue edit --repo unkin/argocd-apps --issue 43 --body "The pipeline fails with ..."
+# prints: #<number> <html_url>
+
 agentpr --version
 agentpr --help
 ```