ci: add merge-blocking markdown format check

[kevmoo]

Pilot for standardizing markdown formatting across the personal repos. This
repo was chosen first because its markdown is already byte-identical to the
standard, so the PR validates the CI wiring against a zero-line formatting
diff -- if it goes red, the workflow is unambiguously at fault.

• .prettierrc.json encodes the mdf standard (prose wrap at 80). Scoped to
  **/*.md via overrides so a stray prettier --write . cannot touch
  pubspec.yaml or workflow YAML. embeddedLanguageFormatting: off stops
  prettier rewriting fenced json/yaml examples.
• .prettierignore excludes generated dartdoc output, which is untracked but
  would otherwise be caught by prettier's filesystem glob and make local runs
  disagree with CI.
• markdown.yml pins prettier to the same version mise installs locally, and
  pins actions by SHA to match the existing convention in ci.yml.

The job is named markdown because the job ID becomes the check-run context
that the branch ruleset matches on. The check is added to the ruleset only
after this lands, so no in-flight PR is stranded by a required context that no
workflow yet produces.

No version bump or changelog entry: this changes CI configuration only and
nothing in the published package.

[github-actions]

📦 Dependency Lower-Bound Validation Summary

✅ dhttpd

• SDK Floor: 3.10.0
• Status: Clean

Dependency	Declared Constraint	Declared Floor	Resolved Version	Status
args	^2.0.0	2.0.0	2.0.0	Exact Floor
build_cli_annotations	^2.0.0	2.0.0	2.0.0	Exact Floor
shelf	^1.0.0	1.0.0	1.0.0	Exact Floor
shelf_static	^1.0.0	1.0.0	1.0.0	Exact Floor

[github-actions]

📊 Cognitive Complexity Analysis

Net Delta: 0 | Added: 0 | Increased: 0 | Improved: 0 | Violations: 0

No modified Dart declarations detected.