refactor(skill): rename validate → web-validate

Clearer scoped name for the W3C + WCAG skill. Updated: folder (git mv),
frontmatter name, H1 title, command refs, CLAUDE.md routing, 6 profiles
(functional — activate the skill by folder name), cross-refs in
harden/seo/depth-matrix/client-handover, agent dispatch refs, README +
USAGE tables.

Confidentiality: the client-deliverable leak-guard regex
(client-handover-writer.md) now matches BOTH /web-validate and legacy
/validate, so older client docs stay covered.

Left intentionally: validator-analyzer agent name (lockstep with
subagent_type + registry), .validate-cache/ + VALIDATE.md (audit-file
family {SEO,GEO,HARDEN,CSO,VALIDATE}.md), .claude/ history (append-only),
CHANGELOG old entry (added a new "renamed" entry instead). NL trigger
keywords kept so "validate" still routes here. Third-party html-validate
untouched.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W9sqAwZxBMZSynZoVrEJhd

CLAUDE.md

@@ -235,7 +235,7 @@ only the non-obvious cases: gstack fallbacks, disambiguation, cryptic names.
 - Architecture review → plan-eng-review
 - Before /clear or /compact → capitalize; end-of-session ritual → close
 - SEO+GEO → seo (GEO only → geo)
-- W3C + WCAG a11y (HTML/CSS validity, axe, pa11y) → validate
+- W3C + WCAG a11y (HTML/CSS validity, axe, pa11y) → web-validate
 - Security audit (secrets, CVE, OWASP) → cso
 - New project → init-project; onboard existing repo → onboard