revert the dark-theme check and doctor probe #1639

Problem

#1626 added a webjs check rule that failed a layout whose dark theme was unreachable, and #1629 moved it into a webjs doctor probe (DARK_THEME_UNREACHABLE) that the scaffold gates at error. The owner does not want a colour-theme convention enforced by the framework's tooling (a fresh app's own CI can fail on it), so the check goes away.

Design / approach

Revert #1629 and #1626 together, so neither the check rule nor the doctor probe nor the scaffold gate remains. The guidance in the docs (light-dark() tokens, the theme script) stays as advice; only the enforcement goes.

Implementation notes (for the implementing agent)

• Where to edit: git revert --no-commit a61c6a6d e78adc07 (newest first). Touches packages/cli/lib/doctor/probes/dark-theme.js, packages/cli/lib/doctor/{codes,runner}.js, packages/server/src/check/{rules-styling,rules,runner}.js, packages/cli/lib/create.js (the DARK_THEME_UNREACHABLE: error gate, ~L588), test/cli/doctor.test.mjs, test/scaffolds/scaffold-template-validation.test.js, and docs (AGENTS.md, packages/cli/AGENTS.md, the skill's built-ins.md and styling.md, website configuration and troubleshooting pages).
• Landmines: later commits may touch the same doc lines (the dedupe revert docs: revert the agent-doc dedupe and restore the full guidance #1637 restored workflow.md/AGENTS.md); resolve toward the pre-feat: webjs check fails a layout whose dark theme is unreachable #1626 text. After the revert, grep for DARK_THEME_UNREACHABLE and dark-theme-unreachable so nothing is left referencing the removed code.
• Tests and docs: doctor tests, scaffold validation tests, webjs check rule list; website docs pages above.

Acceptance criteria

• No DARK_THEME_UNREACHABLE / dark-theme-unreachable reference remains in code, tests, docs or the scaffold's webjs.doctor.gate.
• A fresh scaffold's package.json no longer gates it and passes webjs doctor.
• CI green.