Warn (don't fail) when a feed still points at .example placeholder URLs

[hhkaos]

What happened

ote-template ships ote.config.json with obviously-fake-but-syntactically-valid
placeholder values (feed.url: "https://your-community.example",
feed.organizers[].url likewise) so a fresh fork validates and builds
immediately — see ote-template's README/SETUP.md.
Nothing currently catches an organizer who forgets to replace them: a real
fork published feed.title: "<Your community name> events" to production
with CI green (reported first-hand from ComBuildersES/events, since fixed
by hand on their side).

Proposal

In @opentechevents/validate / build-feed: add a warning (not an
error — must not block a legitimately in-progress feed) when feed.url or
any organizers[].url ends in .example.

.example is RFC 2606-reserved specifically for documentation/placeholder
use and can never legitimately appear in a real published feed, so this is a
clean, low-coupling signal — no need to hardcode any specific template's
placeholder text, and it'll catch this for any template or hand-rolled fork
that follows the same "fake but valid" convention.

Where this surfaces

Once built, this needs zero changes on the ote-template side: it already
calls the validate.yml reusable workflow from ote-tools, so the warning
would show up there automatically for every fork.

Context

Filed as part of a broader fork-lifecycle redesign in ote-template (see
CHANGELOG 0.6.0).
This is one of a few follow-ups intentionally left for ote-tools rather
than solved with logic in the template repo.