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>

.github/skills/impeccable/reference/brand.md

@@ -64,7 +64,7 @@ Brand surfaces have permission for Committed, Full palette, and Drenched strateg
 
 - Name a real reference before picking a strategy. "Klim Type Foundry #ff4500 orange drench", "Stripe purple-on-white restraint", "Liquid Death acid-green full palette", "Mailchimp yellow full palette", "Condé Nast Traveler muted navy restraint", "Vercel pure black monochrome". Unnamed ambition becomes beige.
 - Palette IS voice. A calm brand and a restless brand should not share palette mechanics.
-- When the strategy is Committed or Drenched, the color is load-bearing. Don't hedge with neutrals around the edges — commit.
+- When the strategy is Committed or Drenched, color carries the brand. Don't hedge with neutrals around the edges. Commit.
 - Don't converge across projects. If the last brand surface was restrained-on-cream, this one is not.
 
 ## Layout

.github/skills/impeccable/reference/live.md

@@ -66,7 +66,7 @@ When `screenshotPath` is absent, don't ask for one and don't go looking for the
 
 Reading annotations precisely:
 
-- **Comment position is load-bearing.** Its `{x, y}` is element-local CSS px (same coord space as `element.boundingRect`). Find the child under that point and apply the comment text LOCALLY to that sub-element. A comment near the title is about the title, not a global description.
+- **Comment position carries meaning.** Its `{x, y}` is element-local CSS px (same coord space as `element.boundingRect`). Find the child under that point and apply the comment text LOCALLY to that sub-element. A comment near the title is about the title, not a global description.
 - **Comments and strokes are independent annotations** unless clearly paired by overlap or tight proximity. Don't let the visual weight of a prominent stroke override the precise location of a textually-specific comment elsewhere.
 - **Strokes are gestures — read them by shape.** Closed loop = "this thing" (emphasis / focus); arrow = direction (move / point to); cross or slash = delete; free scribble = emphasis or delete depending on context. A loop around region X means "pay attention to X," not "only change pixels inside X."
 - **When a stroke's intent is ambiguous** (circle or arrow? emphasis or move?), state your reading in one sentence of rationale rather than silently guessing. If the uncertainty materially changes the brief, ask one short clarifying question before generating.

.github/skills/impeccable/reference/personas.md

@@ -130,7 +130,7 @@ Test the interface through the eyes of 5 distinct user archetypes. Each persona
 - Are primary actions in the thumb zone (bottom half of screen)?
 - Is state preserved if the user leaves and returns?
 - Does it work on slow connections (3G)?
-- Can forms leverage autocomplete and smart defaults?
+- Can forms use autocomplete and smart defaults?
 - Are touch targets at least 44×44pt?
 
 **Red Flags** (report these specifically):

.github/skills/impeccable/reference/typeset.md

@@ -109,7 +109,7 @@ Build a clear type scale:
 - **Performance**: Are web fonts loading efficiently without layout shift?
 - **Accessibility**: Does text meet WCAG contrast ratios? Is it zoomable to 200%?
 
-Remember: Typography is the foundation of interface design — it carries the majority of information. Getting it right is the highest-leverage improvement you can make.
+Remember: Typography is the foundation of interface design. It carries most of the information on the page. Getting it right changes the design more than any other single move.
 
 ## Live-mode signature params