v0.2.1: arg validation, RDAP banner fix, demo gif

[sitapix]

Summary

Tighten CLI input handling, fix a TTY rendering collision, add a README
demo, and wire up markdown linting so docs stay clean going forward.

Changes

• dibs verify mybrand now errors with a shell-safe suggestion
  (did you mean: dibs --verify "mybrand") instead of silently dropping
  the second arg. Values with whitespace are %q-quoted so the
  suggestion round-trips through a shell.
• RDAP banner moved into a new Renderer.BeginVerification(count) hook.
  Terminal clears its in-place progress bar before printing so the two
  don't collide on the same line. CSV/JSON treat it as a no-op.
• colorGreen switched from ANSI 32 to 92 so available-domain lines
  stay legible on muted themes (TokyoNight, Solarized).
• Fixed goreportcard misspell flag in config_test.go.
• README now leads with a vhs-rendered demo GIF and carries release /
  Go Reference / Report Card badges. CONTRIBUTING documents how to
  regenerate the demo from demo/demo.tape.
• Added markdownlint-cli2 with a line_length: 85 config, a
  make docs-lint target, and wired it into make release-check.
  Reflowed all prose in README, CONTRIBUTING, and CODE_OF_CONDUCT to
  comply.

Type of Change

• Bug fix
• New feature
• Documentation
• Refactoring
• Other: tooling (markdownlint)

Testing

• go test ./... passes (174 tests, -race clean)
• go vet ./... passes
• make release-check fully green (fmt, vet, lint, markdownlint,
  race, deadcode, govulncheck, mod verify/tidy, reproducible build)
• Tested manually on macOS / Linux

Checklist

• Code follows project style
• Tests added for new functionality
  (TestRejectsExtraPositionalArgs,
  TestTerminalRenderer_BeginVerificationClearsProgressBarOnTTY)
• Documentation updated (README, CONTRIBUTING)