docs: fix README anchor and clarify demo commands

[takagibit18]

Problem

While reading through the docs, I noticed a couple of small things that could make the first-run experience a little confusing:

• The "Webwright plugin/skills" link in the README pointed to a section anchor that no longer exists.
• The README flags table did not mention the existing --debug option.
• The Task Showcase README showed python app.py, but it was not immediately clear whether that command should be run from the repo root or from the assets/task_showcase/ directory.

Changes

This PR makes a few small documentation updates:

• Fixes the README internal link so it points to the existing "Use as a Plugin" section.
• Adds --debug to the README flags table.
• Clarifies how to run the Task Showcase demo from either the repository root or the assets/task_showcase/ directory.

Validation

• Reviewed the diff to confirm the changes are documentation-only.
• Ran git diff --check.

No runtime tests were run because this PR only updates documentation.

[takagibit18]

@microsoft-github-policy-service agree