post-V0: customize mdBook theme to use plumb tokens for full 0-violation dogfood goal

[aram-devdocs]

Tracks the original PRD §15 / runbook acceptance of '0 violations dogfooding clean' on https://plumb.aramhammoudeh.com/. V0 ships with the relaxed acceptance ('exit 0 or 3, deterministic') because mdBook's default theme drifts from plumb's stock tokens. Real fix: customize the mdBook theme via book.toml + docs/theme/css/variables.css wiring CSS custom properties to plumb's [spacing]/[color]/[type] defaults.

Acceptance: plumb lint https://plumb.aramhammoudeh.com/ exits 0 with no warnings/info either, after the theme is aligned.