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.

.claude/memory/learnings.md

@@ -147,6 +147,7 @@ rules:
 | LRN-155 | 2026-09-16 | ask under auto: doc says prompt, probe on 2.1.273 says no; re-probe after upgrades | any permission-tier reasoning |
 | LRN-156 | 2026-09-16 | autoMode.allow = exception tier; static interpreter allow suspended under auto → conditions live in prose | conditional permissions |
 | LRN-157 | 2026-09-16 | gap-only trigger blind to taste → add a trigger class, not budget; ask at plan, mid-run for leftovers | any "ask more" request |
+| LRN-158 | 2026-09-22 | Installer refusing symlinked paths vs a symlinked config dir → stage under a throwaway HOME, move the result | any vendor installer writing into ~/.claude or ~/.config |
 
 ---
 
@@ -1490,3 +1491,11 @@ Rule: when editing a doctrine file under structure locks, grep the test's lock s
 - **Pattern**: a trigger that fires only on missing outcome / scope / constraints lets every taste choice through — "add a share icon" is complete by those criteria and the icon's side is decided downstream. More budget changes nothing; the fix is a new trigger class (VISIBLE / PUBLIC NAME / SCOPE). Cost geometry: a fresh re-dispatch keeps the working tree and loses the executor's reasoning → the same question costs about one executor run more mid-run than at PLAN. So: sweep once at the plan step, keep the mid-run channel for leftovers. Executor tags the class; orchestrator re-reads it (tag = hint, a mis-tag would offload class 4 onto the human). Relayed questions obey [[LRN-102]]: context inside `AskUserQuestion`, nothing the user needs printed before it.
 - **Future application**: any "ask more" request → check WHICH trigger is blind before touching a quota. Any orchestrator with a "decide it yourself" fallback on an executor halt → route by class first.
 - **Reference**: [[BDR-091]], `lib/contract-interview.md` STEP 2 + MID-RUN CLARIFICATION.
+
+## LRN-158 — A hardened installer + a symlinked config dir = documented command fails; stage under a throwaway HOME
+- **Date**: 2026-09-22
+- **Context**: `21st install-skill` (= `21st skills install --global`) is upstream's documented one-liner. Here it dies: `Refusing to access symbolic link /home/…/.claude/skills`. The installer walks every segment of `<HOME>/.claude/skills/<n>/SKILL.md` with an `assertNoSymlinkComponents` guard (anti symlink-escape); this repo's whole model is `~/.claude/skills -> repo/skills`. Two correct designs, mutually exclusive on the same path.
+- **Pattern**: don't fight the guard and don't unlink the config dir. Run the installer with `HOME=$(mktemp -d)` so it writes into a pristine real tree, then move the output to the vendored dir the repo controls and symlink from there. Same shape as the impeccable/ctx7 staging (`mktemp -d`, install, `mv` into `skills-external/`), with HOME as the extra lever. Two conditions make it safe: the command must need nothing else from HOME (checked: manifest + content fetch are unauthenticated, hash-verified), and the moved payload must be self-contained.
+- **Also**: read the npm tarball, not the vendor's web page. 21st.dev's `/mcp` and `/llms.txt` still document the MCP `init --client` flow with an API key; the package README states the CLI supersedes it. `curl registry.npmjs.org/<pkg>` + untar + read `README.md`/`dist` answered every question (commands, exit codes, where files land) that the site got wrong.
+- **Future application**: any vendor installer that writes into `~/.claude`, `~/.config` or `~/.agents` on this machine. Probe first with a fake HOME containing the symlink, before wiring it into `install-plugins.sh` — the failure is instant and unambiguous.
+- **Reference**: [[BDR-093]], `install-plugins.sh` Step 8.7, `update-all.sh` 7.4. Links [[LRN-034]] (run the real thing), [[BLK-014]]-class symlink/self-heal issues.