feat(21st): replace the magic MCP with the @21st-dev CLI + skill pack

Upstream supersedes `@21st-dev/magic` with `@21st-dev/cli` (bin `21st`):
same endpoint, `21st login` in place of an API key, no MCP process loaded
into every session.

- install-plugins.sh Step 8.7: `npm i -g @21st-dev/cli` (pinned in
  plugins.lock.json), staged `21st skills install`, TTY-only login offer,
  pack disabled by default. update-all.sh 7.4 refreshes both.
- The documented `21st install-skill` cannot be used: the installer refuses
  to follow a symlink on the target path and `~/.claude/skills` is one. The
  install runs under a throwaway HOME and the result moves into
  skills-external/21st-* (gitignored), symlinked on demand.
- toggle-external.sh manages `21st` as a pack (names globbed from
  skills-external/21st-*, parked under plain names). `magic` is gone.
- The 5 design skills join design/web/web-full/full and MANAGED_EXTERNALS;
  21st-registry and 21st-design-sync stay parked. MANAGED_MCPS is now empty
  and profile.sh's dead magic branches are removed.
- Design gate: GATE-BLOCK gains `21st` (required-manual, magic's old slot)
  and `21st-ui-build`; PATH repair extended to the npm global bin.
- settings.json: the 4 mcp__magic__* ask entries go; the outward-facing
  21st verbs land in autoMode.soft_deny, the tier that holds under auto
  mode (LRN-153).
- Docs: README, CLAUDE.global.md, design-gate.md, profile SKILL.md,
  .env.example, .gitleaks.toml, link.sh. BDR-093, LRN-158.

Tests: profile-set-managed 17/17, make test green except 2 pre-existing
gitflow FAILs (gitleaks binary absent on this host), shellcheck clean.

.env.example

@@ -1,9 +1,9 @@
 # Local secrets for Claude Code plugin install scripts.
 # Copy to ~/.claude/.env and fill in real values. link.sh symlinks repo/.env to it; the secret never enters git.
 #
-# Used by: lib/toggle-external.sh enable|disable magic
-# Get a key at: https://21st.dev/magic (dashboard → API keys)
-MAGIC_API_KEY=your_21st_dev_magic_api_key_here
+# 21st.dev needs nothing here since 2026-09-22: the Magic MCP server was
+# replaced by the `21st` CLI, whose auth is `21st login` (browser token in
+# ~/.config/21st). Any leftover MAGIC_API_KEY line is dead — delete it.
 
 # ── Google SEO data layer (lib/seo-data) — used by /seo FULL ──
 # OAuth Desktop client: GCP console → APIs & Services → Credentials → OAuth client (Desktop).