ci: check Markdown links on pull requests and main

[altaywtf]

Adds a Links workflow that runs the shared offline Markdown link and anchor check (frontend-links.yml v1.1.0) on every pull request and on main.

• No path filter, so Markdown-only pull requests run it too. Pinned to the v1.1.0 commit with the tag as its comment; Dependabot's github-actions updates move it.
• The same lychee 0.24.2 command run locally on this branch: 0 errors across 10 tracked Markdown files; with an injected missing file and missing anchor it exits 2.
• actionlint and zizmor 1.29.0 report nothing for the new workflow.
• Distribution now lists the caller.

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Written by an agent (Claude Code, Claude Opus 5.5)

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