Add option to format summary as Markdown or HTML

This would be more visually appealing when experiment results are viewed in a browser (e.g., in GitHub Actions summaries).

For example, instead of:

Performance Characteristics
---------------------------
Initial build time: 6m 28.381s
Build time with instant savings: 38.472s, 5m 49.909s savings
Build time with pending savings: 38.472s, 0.000s additional savings
Avoided cacheable tasks: 15 tasks, 2m 4.706s total saved execution time
Executed cacheable tasks: 3 tasks, 25.000s total execution time
Executed non-cacheable tasks: 148 tasks, 28.253s total execution time
Serialization factor: 1.86x first build, 1.97x second build

Not all cacheable tasks' outputs were taken from the build cache in the second build. This reduces the savings in task execution time.

See https://gradle.com/bvs/main/Gradle.md#performance-characteristics for details.

---

Markdown could be used instead:

Performance Characteristics
---------------------------

| | |
|---|---|
| Initial build time: | 6m 28.381s |
| Build time with instant savings: | 38.472s, 5m 49.909s savings |
| Build time with pending savings: | 38.472s, 0.000s additional savings |
| Avoided cacheable tasks: | 15 tasks, 2m 4.706s total saved execution time |
| Executed cacheable tasks: | 3 tasks, 25.000s total execution time |
| Executed non-cacheable tasks: | 148 tasks, 28.253s total execution time |
| Serialization factor: | 1.86x first build, 1.97x second build |

> [!WARNING]
> Not all cacheable tasks' outputs were taken from the build cache in the second build. This reduces the savings in task execution time.

See the [Performance Characteristics](https://gradle.com/bvs/main/Gradle.md#performance-characteristics) section in the README for details.

Start by locating the code that assembles the experiment performance summary and the entry points that consume it. Review how the current plain-text output is produced, then define where a Markdown or HTML format option belongs. Done means the selected format renders the summary, table, warning, and link appropriately for browser viewing.

Written by the indexing model from the issue text.