chore(deps): remove unmaintained broken-link-checker dev dependency

[DeepDiver1975]

What

Removes the unmaintained broken-link-checker dev dependency and the npm run linkcheck script.

Why

broken-link-checker@^0.7.8 was last released around 2017 and is effectively unmaintained. It pulls in a large legacy transitive tree (deprecated nopter, urlobj, urlcache, uuid@2 which relies on Math.random, an old chalk/ansi stack) — a needless security and maintenance liability that keeps generating Dependabot noise.

It was only ever invoked through the linkcheck npm script and is not used in CI. Local link checking remains fully possible via the better-maintained tools already documented in the docs repository (Antora xref-validator, htmltest, filiph's linkcheck).

Changes

• Drop the broken-link-checker devDependency
• Drop the now-unused linkcheck npm script
• Repoint agent/CLAUDE docs that referenced it to the documented alternatives (where present)
• Regenerate package-lock.json (removes ~19 transitive packages)

No replacement dependency is added (per the repo policy of not introducing new dependencies without discussion).

Verification

• npm install succeeds; the lockfile no longer contains any broken-link-checker family packages
• npm run antora still builds the docs (a devDependency removal does not affect the build; verified end-to-end)
• CI is unaffected (no workflow used linkcheck)

🤖 Generated with Claude Code

[DeepDiver1975]

🤖 Automated review by Claude Code review agent.

LGTM — clean removal of the unmaintained broken-link-checker dev dependency.

Verified:

• package.json: broken-link-checker devDep and the linkcheck script both removed.
• package-lock.json: the broken-link-checker node and transitive deps pruned. The diff also shows Git conflict markers (<<<<<<< / ======= / >>>>>>> from an earlier "remove dead Elasticsearch" change) being removed — those markers exist only in the base branch; the PR-head lockfile is conflict-marker-free and valid JSON, so this PR also tidies up that pre-existing breakage. Nice.
• No dangling references — grepped package.json, lockfile, docs, and .github/workflows/ci.yml for linkcheck / broken-link-checker / blc: none remain.
• CI: Build documentation and lint both green (confirms the resulting lockfile installs cleanly).
• No CHANGELOG in this repo (n/a).

No blocking issues.