fix(board-docs): point homepage_url at board.ycst.org.uk

[robinbowes]

The docs site was renamed from admin.ycst.org.uk to board.ycst.org.uk — the
same rename the repo itself went through as ycst-admin-docs -> board-docs.
homepage_url was left pointing at the old host, which has no DNS record at all
(the apex ycst.org.uk resolves to Krystal; the admin. subdomain does not).
board.ycst.org.uk resolves to the same host and serves.

Plan: 0 to add, 1 to change, 0 to destroy — homepage_url on
github_repository.this only.

The historical spec and plan documents still name the old host. They are left
unedited as records of what was believed at the time; the correction is noted in
decisions/2026-08-04-ycst-admin-docs-private-cpanel.md.

🤖 Generated with Claude Code

https://claude.ai/code/session_01C15usdmazQC71xZC1YFGHg

[yo61-lastlight]

Corrects homepage_url on board-docs from the dead admin.ycst.org.uk host to board.ycst.org.uk, matching the repo/site rename already documented; the decision doc is updated with the same correction and the historical record is left intact. Minimal, well-justified, no functional concerns.