Vercel's WAF denies some residential IP ranges for the whole docs domain,
which leaves the Bot Mode Skills Hub picker on a 403 block page with no
alternate origin (#118203). Probe the primary picker URL before each
browse session and fall back to the equivalent GitHub Pages deployment
when it is unreachable, refuses us, or hangs past an 8s deadline; picker
installs are accepted from either trusted origin but still gated on our
own frame's contentWindow. Documented as an FAQ troubleshooting entry.
Re-lands the surviving half of #118206 (its Capabilities picker target
was since deleted by the native catalog).
Fixes#118203
Co-authored-by: KoNit-K <konit.block@protonmail.com>