docs(skills): clarify Lighthouse score scale and contrast workflow (#3006) (#3218)

Fixes #3006.

In `skills/a11y-debugging/SKILL.md`:
- Clarify in Step 1.2 ("Analyze the Summary") that `lighthouse_audit`
default text output formats category scores on a 0-100 scale followed by
`Passed: N` and `Failed: N` audit counts. The 0-1 scale
(`summary.scores[].score`) and `summary.audits.failed` are present in
structured content when `--experimentalStructuredContent` is used.
- In Step 7 ("Color Contrast"), direct agents to use the "Check Color
Contrast" snippet via `evaluate_script` directly, clarifying that
`list_console_messages` with `types: ["issue"]` only surfaces issues
already emitted by Chrome and does not request a contrast check.

## Testing

- Ran `npx prettier --check skills/a11y-debugging/SKILL.md` (passes
cleanly).
- Verified `npm run build` and `npm run typecheck` (`tsc --noEmit`) pass
with 0 errors.

---------

Co-authored-by: Alex Rudenko <alexrudenko@chromium.org>

skills/a11y-debugging/SKILL.md

@@ -21,8 +21,8 @@ Start by running a Lighthouse accessibility audit to get a comprehensive baselin
     - Set `mode` to `"navigation"` to refresh the page and capture load issues.
     - Omit `outputDirPath` to save reports to the default temporary directory (the exact path to `report.json` is returned under `Reports`), or set `outputDirPath` to a directory within the workspace.
 2.  **Analyze the Summary**:
-    - Check `scores` (0-1 scale). A score < 1 indicates violations.
-    - Review `audits.failed` count.
+    - In the default text output, category scores use a 0-100 scale (100 is a perfect score), followed by `Passed: N` and `Failed: N` audit counts.
+    - If the server was started with `--experimentalStructuredContent`, inspect `summary.scores[].score` (0-1 scale) and `summary.audits.failed` in the structured content instead.
 3.  **Review the Report (CRITICAL)**:
     - **Parsing**: Do not read the entire file line-by-line. Use a CLI tool like `jq` or a Node.js one-liner to filter for failures from the report path returned by `lighthouse_audit`:
       ```bash
@@ -73,12 +73,7 @@ _Pass the element's `uid` from the snapshot as an argument to `evaluate_script`.
 
 ### 7. Color Contrast
 
-To verify color contrast ratios, start by checking for native accessibility issues:
-
-1.  Call `list_console_messages` with `types: ["issue"]`.
-2.  Look for "Low Contrast" issues in the output.
-
-If native audits do not report issues (which may happen in some headless environments) or if you need to check a specific element manually, use `evaluate_script` with the **"Check Color Contrast" snippet** found in [references/a11y-snippets.md](references/a11y-snippets.md).
+To verify color contrast ratios, use the **"Check Color Contrast" snippet** found in [references/a11y-snippets.md](references/a11y-snippets.md) with `evaluate_script`.
 
 ### 8. Global Page Checks