docs: strip AI prose, add STYLE.md and validateProse (#134)

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>

site/content/tutorials/iterate-live.md

@@ -47,7 +47,7 @@ A few things you can do before pressing Go:
 
 - **Click the command chip** (default is `impeccable`, the freeform action). Pick a specific action like `bolder`, `delight`, `layout`, or `typeset` to constrain the variants along one dimension.
 - **Type in the freeform field.** "More playful." "Less SaaS." "Feel like a newsletter from a magazine."
-- **Drop a comment pin** by clicking anywhere on the picked element. The pin's position is load-bearing: a comment near the title is about the title, not the whole element.
+- **Drop a comment pin** by clicking anywhere on the picked element. The pin's position matters: a comment near the title is about the title, not the whole element.
 - **Draw a stroke** by dragging across the element. Closed loop = "this part matters." Arrow = direction. Cross = "delete this." The skill reads strokes by shape, not by pixel content.
 
 When the brief feels clear, hit **Go**.