Add a real spacing scale: internal <= external, not habit

[codeswithroh]

What this changes

Replaces the thin "8px base grid" note with a proper spacing system, grounded in real research: Tailwind's default 4px base unit, the Material/Apple 8pt grid convention, and the internal padding <= external gap Gestalt proximity rule — the actual mechanism behind the cramped pricing cards and imbalanced sections in the real screenshots that prompted #26.

• references/style-tokens.md — new ## Spacing scale section: a space-1..space-24 token table, the internal<=external rule with a worked pricing-card example, card-padding minimums by type (compact/content/showcase), section-level padding guidance, and the existing radius scale kept in place.
• references/style-lock-format.md — ## Density & spacing expanded to record the actual chosen tokens per role, not just a base unit.
• SKILL.md — sixth non-negotiable added to Step 4: "Spacing follows the scale, not habit," citing the rule and the space-6/24px content-card floor.
• references/anti-slop-checklist.md — new checklist item making the rule a specific, checkable failure ("card padding larger than the gap around it") instead of a vague "needs more space" feeling.

Built on top of #25 (App shell section) since this content cross-references it — merged that first and rebased this branch so the references resolve to real content, not a section that didn't exist yet.

Also swept 7 stray "preset" mentions found while re-reading these files — leftovers from earlier cleanup passes that only touched README/CONTRIBUTING/site HTML and missed style-tokens.md, style-lock-format.md, anti-slop-checklist.md, SKILL.md, the PR template, and site/README.md.

Closes #26

Test plan

• grep -rn "preset" across *.md/*.html returns nothing
• Headings and cross-references in all four edited reference files verified against actual post-rebase content (no dangling links to sections that don't exist)
• SKILL.md Step 4 non-negotiables renumbered 1-6, sequential and correct
• Markdown code-fence balance checked in all edited files

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