[autofix] pixiv/download: full artwork URL rejected

## Summary
OpenCLI autofix repaired this adapter locally, and the retry passed.

## Adapter
- Site: `pixiv`
- Command: `pixiv/download`
- OpenCLI version: `1.8.6`

## Original failure
- Error code: `COMMAND_EXEC`

~~~
Invalid illustration ID: https://www.pixiv.net/artworks/141704294
~~~

## Root cause
The positional argument was validated as digits only even though users commonly provide canonical Pixiv artwork URLs.

## Local fix summary
Normalize either a numeric ID or a trusted `pixiv.net/artworks/` URL to the numeric illustration ID before calling `/ajax/illust//pages`. Invalid values now raise `ArgumentError`.

## Verification
The full URL downloaded all 10 pages successfully. All files were valid 1536x1536 JPEG images, totaling about 3.9 MB. `opencli validate pixiv/download` passed.

Start at the pixiv/download command entry point and trace how its positional argument is validated before the Pixiv page request. Run the command validation and the reported full-URL case, then verify that numeric IDs and trusted pixiv.net/artworks/ URLs work while invalid values raise ArgumentError and the download completes.

Written by the indexing model from the issue text.