docs: strip AI prose, add STYLE.md and validateProse

[pbakaus]

Site copy was being called out as AI slop (specifically the word
"load-bearing"). Five-pass cleanup with a build validator to keep it
from creeping back.

Pass 1 — mechanical purge:

• Remove "load-bearing" from impeccable.md, brand.md, live.md,
  iterate-live.md
• Remove "highest-leverage" from critique.md, typeset.md, designing
• Remove em dashes from all 9 slop-page rule cards
• Replace "leverage" verb in personas.md

Pass 2 — rewrite the worst offenders:

• Changelog v2.0 "Data-driven skill rewrite" entry: drop "data-driven",
  "frontier models", "collapses into monoculture", "biggest unlock",
  "reflex defaults"; name the actual mechanism
• README opener: drop "deeper expertise and more control"; replace with
  three concrete differentiators (7 reference files, 23 commands, 27
  detection rules)
• Neo Mirai case study opener: action-first, name the image model used

Pass 3 — editorials:

• Fix negation pivot in distill.md ("simplicity is not about ... It is
  about ...")

Pass 4 — homepage why-panels:

• Foundation lead: name the 7 reference files specifically
• Language lead: show the discipline mapping with real command names
• Production-codebases panel: drop "Impeccable isn't a sketchpad"
  negation pivot
• Ships-code panel: replace "is native to that world" with "runs there"

Pass 5 — STYLE.md and validator:

• New STYLE.md at root: editorial brief with 12 principles and the
  enforced denylist (each rule with a rationale and a suggested
  replacement)
• scripts/build.js: validateNoEmDashes becomes validateProse. Adds 21
  phrase rules with rationales, catches the `--` em-dash substitute,
  expands target list to site/pages, site/content, README.md,
  README.npm.md
• CLAUDE.md: replace the em-dash section with a STYLE.md pointer and
  document the two-content-tree footgun (content/site/ vs site/content/
  must be edited in lockstep until they're unified)

Co-authored-by: Claude noreply@anthropic.com

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Note

Medium Risk
Moderate risk because scripts/build.js now enforces new prose rules across multiple content trees/READMEs, which can fail builds unexpectedly if future copy trips the denylist.

Overview
Adds a new root STYLE.md editorial brief (principles + enforced denylist) and updates CLAUDE.md to point authors at it and document the dual content/site vs site/content editing requirement.

Replaces the build’s em-dash-only check with validateProse in scripts/build.js, expanding scan targets (including site/pages and READMEs) and failing the build on em dashes, -- substitutes, and a denylisted set of common AI/marketing phrases with per-rule rationales.

Sweeps user-facing docs and site pages to remove/replace flagged terms (e.g. load-bearing, highest-leverage, data-driven) and to rewrite several introductions/blurbs (README, homepage sections, case study copy, changelog entry, tutorials) to be more concrete and less template-like.

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