ci: update to rust version of flint

[zeitlinger]

This updates the existing link linting to the current Rust-based flint release.

What changes in this PR:

• install flint from github:grafana/flint = "0.20.3"
• replace the old contrib-hosted lint:links shell tasks with flint run
• keep the existing Lychee configuration under .github/config
• enable check_all_local = true so local links are checked across the repo
• preserve full-repo link checking on pushes by keeping mise run lint --full on the non-PR workflow path

Why this stays small:

• this repo does not currently carry Markdown lint tooling to migrate here
• the goal is only to replace the old link-check task path with the released Rust flint path

Verification:

• mise run lint
• mise run lint --full

[codeboten]

the changes look ok. i'm not super familiar w/ flint... was there a discussion about switching to it for link checking somewhere?