feat(renderer): wrap step timeline in details element

[Copilot]

Step timeline now renders as a collapsible <details> element instead of an always-visible heading, reducing visual clutter in workflow summaries.

Changes

• Modified generateStepAnnotations() to wrap output in <details>/<summary> tags
• Updated test expectations for HTML structure

Implementation

-return `\n#### Step Timeline\n\n${annotations.join("\n")}`;
+return `\n<details>\n<summary>Step Timeline</summary>\n\n${annotations.join("\n")}\n</details>`;

Timeline is collapsed by default; users expand it on demand.