docs: fix execution file parsing example

[looooown2006]

Summary

Fixes the base-action/README.md example for parsing execution_file.

The previous example looked for executionLog[i].role === 'assistant', but the execution log contains SDK events with top-level type fields. Assistant content is nested under message.content, and successful runs also include a final type: "result" event.

This updates the example to:

• prefer the final result event's result string;
• fall back to the last assistant text block under message.content;
• document the existing structured_output and session_id outputs in the base action Outputs table;
• point typed automations toward --json-schema + structured_output instead of parsing the full execution log.

Tests

• npx prettier@3.5.3 --check base-action/README.md
• git diff --check

Note: I did not run bun test / bun run typecheck because Bun is not installed in this local environment. This is a README-only documentation change.

Closes #1296.