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 remote lint:links shell task 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 on push and scheduled runs so link checking still scans the full repo outside pull requests

Why this stays small:

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

Verification:

• mise run lint
• mise run lint --full

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.25%. Comparing base (5f5b054) to head (cf1d2a4).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8315   +/-   ##
=========================================
  Coverage     90.25%   90.25%           
  Complexity     7691     7691           
=========================================
  Files           850      850           
  Lines         23203    23203           
  Branches       2356     2356           
=========================================
  Hits          20943    20943           
  Misses         1532     1532           
  Partials        728      728           

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[jack-berg]

Can you connect the dots for me here..

My read:

• mise is ageneric task runner
• lint is a task for mise defined in the repo to call fling run
• Flint run is pluggable, lychee plugs in as a linter for flint - how exactly?