tooling: scheduled external/dead-link monitoring (lychee)

[VijitSingh97]

Part of the v1.1 site-tooling set · companion to #11.

CI's htmltest runs with CheckExternal: false (.htmltest.yml) — a deliberate choice so PR CI stays deterministic and never fails on a third-party outage. The trade-off: no job ever checks outbound links, so link rot to getmonero.org, GitHub, Tari, etc. goes unnoticed.

Proposal

Add a scheduled (e.g. weekly cron + workflow_dispatch) link-check against the built public/ that opens/updates a tracking issue on breakage — keep it out of the PR gate so determinism is preserved.

• lycheeverse/lychee-action is the standard fit (redirects, rate-limit backoff, retries, .lycheeignore).
• Alternatively, reuse htmltest with CheckExternal: true in a separate scheduled config.

Acceptance

• Scheduled workflow checks external links on the built site.
• Broken links open/append to a tracking issue (never PR-blocking).
• .lycheeignore (or equivalent) for known auth-walled/flaky hosts, documented.