Add missing ref and deadline fields to issue responses (#169)

## Summary

### Read (issue responses)
- Add `ref` (branch) field to issue responses when non-empty
- Add `deadline` (due_date) field to issue responses when non-nil
- Applied to `slimIssue()`, `slimIssues()` in `operation/issue/slim.go` and `slimIssues()` in `operation/search/slim.go`

### Write (issue_write)
- Add `ref` parameter to `issue_write` tool for both `create` and `update` methods
- Allows setting the branch reference on issues via the MCP, consistent with the SDK's `CreateIssueOption.Ref` and `EditIssueOption.Ref` fields

Fixes #168

## Test plan
- [ ] `go test ./...` passes
- [ ] Verify `issue_read` returns `ref` when a branch is assigned to an issue
- [ ] Verify `issue_read` returns `deadline` when a due date is set
- [ ] Verify `list_issues` and `search_issues` include these fields
- [ ] Verify `issue_write` with method `update` can set `ref` on an issue
- [ ] Verify `issue_write` with method `create` can set `ref` on a new issue

Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/169
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>
Co-authored-by: pengu <jeremy@wenjy.fr>
Co-committed-by: pengu <jeremy@wenjy.fr>

operation/issue/slim.go

@@ -63,6 +63,12 @@ func slimIssue(i *gitea_sdk.Issue) map[string]any {
 			"title": i.Milestone.Title,
 		}
 	}
+	if i.Ref != "" {
+		m["ref"] = i.Ref
+	}
+	if i.Deadline != nil {
+		m["deadline"] = i.Deadline
+	}
 	if i.PullRequest != nil {
 		m["is_pull"] = true
 	}
@@ -88,6 +94,12 @@ func slimIssues(issues []*gitea_sdk.Issue) []map[string]any {
 		if len(i.Labels) > 0 {
 			m["labels"] = labelNames(i.Labels)
 		}
+		if i.Ref != "" {
+			m["ref"] = i.Ref
+		}
+		if i.Deadline != nil {
+			m["deadline"] = i.Deadline
+		}
 		out = append(out, m)
 	}
 	return out