feat(scenario): let context.downloadFile save data: URLs as run artifacts

Scenario steps can now persist in-script generated content (e.g. an
obtained VC's JWT) as a run file via context.downloadFile('data:...'),
without needing a network fetch. Bumps to 1.8.0.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/scenario.md

@@ -36,6 +36,8 @@ Available in scope:
 - `context.dumpDom(selector?)`: simplified DOM snapshot
 - `context.log(...args)`, `context.warn(...args)`, `context.error(...args)`: structured step logs
 - `context.runSnippet(name, ...args)`: execute stored snippet code with the same context
+- `context.getScenarioFiles(opts?)`: list files uploaded to the scenario (`limit`, `offset`)
+- `context.downloadFile(url, opts?)`: fetch `url` and save the result as a run artifact (requires a real scenario run — throws when invoked ad hoc, e.g. via the `exec_code` MCP tool). `opts` may include `method`, `headers`, `body`, `filename`. `url` also accepts a `data:` URI (`data:<mediaType>;base64,<data>` or `data:<mediaType>,<percent-encoded data>`) to save content generated in-script (e.g. a credential JWT) directly, without an actual network fetch.
 - `console`: proxied to run logs (`log`, `warn`, `error`, etc.)
 
 `validateCode` additionally receives `result` in scope, which is the value returned by `execCode`.