feat: generate downloadable ADaM XPT + specs artifact on every PR

[Copilot]

Users need a convenient way to download the latest generated XPT files and spec sheets from each PR to validate them in Pinnacle 21 Community.

New workflow: .github/workflows/adam-artifacts.yml

• Triggers on all PR events (opened, synchronize, reopened, ready_for_review) targeting main
• Runs all 8 ADaM R scripts in a single R session (ADSL first to satisfy any downstream dependencies); script errors are caught per-script so a single failure doesn't abort the rest
• Collects outputs — XPT files found under /tmp + spec files (*.xlsx, *.csv) from metadata/
• Uploads as a single zip artifact named adam-xpt-and-specs (14-day retention)
• Posts a sticky PR comment linking directly to the Actions run where the artifact can be downloaded

Artifact contents per run:

adam-xpt-and-specs.zip
├── adsl.xpt
├── adae.xpt
├── advs.xpt
├── adrs.xpt
├── adtte.xpt
├── adpc.xpt
├── adppk.xpt
├── ader.xpt   # also adee, ades, adtrr
├── safety_specs.xlsx
├── pk_spec.xlsx
├── onco_spec.xlsx
└── sdtm_ct.csv

R dependency installation uses caching (cache: true) to keep re-runs fast.

[bms63]

I'm not 100% what I am doing with this - but I wanted a way to easily grab xpts and specs and test in P21 Community. I can't do this for every PR, but when I get an itch to look at stuff I can easily use. Perhaps I can even write a way for folks to check this then using their own local copy of P21C - maybe this could become a standard tempalte checklist item down the road.

[github-actions]

The website is available at https://pharmaverse.github.io/examples/copilot/add-downloadable-artifact-zip.