ci: scheduled external-link monitor with lychee (closes #12)

[VijitSingh97]

Resolves #12. CI's htmltest runs with CheckExternal: false (deterministic — never flakes on a third-party outage), so nothing ever checks outbound links. Link rot to getmonero.org / GitHub / Tari goes unnoticed.

What this adds

• .github/workflows/links.yml — a weekly (+ workflow_dispatch) job that builds the site and runs lychee over ./public, excluding our own domain (internal links stay htmltest's job). fail: false, so it never blocks a PR. On breakage it opens — or updates — a single Broken external links (lychee) tracking issue via gh (deduped by exact title).
• .lycheeignore — documented placeholder for flaky/auth-walled hosts; none needed yet since the site only links to stable hosts.

The Hugo install reads .hugoversion when present (falls back to 0.162.1), so it auto-tracks the single-source pin from #15 once that merges.

Acceptance (#12)

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

Verified locally

• lychee --dump over a production build lists exactly the outbound links (github repos, getmonero.org, tari.com) with the own-domain internal links correctly excluded
• prettier / YAML / zizmor all clean

Note: it's a scheduled workflow, so it doesn't run on this PR — verification is the local lychee --dump + static checks above. Trigger it manually via the Actions tab (workflow_dispatch) after merge to see a first run.

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