workstation-v0 GNOME dock extension validation helper

[mdheller]

Closes #130.

What changed:

• Added profiles/linux-dev/workstation-v0/bin/check-gnome-dock-extension.sh.
• Added .github/workflows/workstation-gnome-dock-validation.yml.
• Documented the helper in profiles/linux-dev/workstation-v0/gnome/README.md.

Validation evidence:

• bash -n profiles/linux-dev/workstation-v0/bin/check-gnome-dock-extension.sh
• bash profiles/linux-dev/workstation-v0/bin/check-gnome-dock-extension.sh
• workflow smoke asserts required keys and a positive stubbed path

Known gaps:

• This helper is read-only and does not install or enable extensions.
• It does not alter the installed extension set.

Non-goals preserved:

• No package manifest changes.
• No GNOME Shell replacement.
• No runtime mutation.