feat: Claude auto-update, spec workflow overhaul, share URL shortener

* `pilot update` (alias `pilot upgrade`) now updates Claude Code alongside
  Pilot Shell via npm registry lookup
* branch isolation toggle replaces worktreeSupport, dispatcher skips branch
  question when disabled, Settings page restructured into left-vertical-tab
  sidebar with extracted section components
* spec skills step count cut 60→42 (-30%) by merging tiny conditional steps;
  eliminates user-reported phantom Step 1 → Step 4 gaps; behavior preserved
* /spec gains stop-guard objective re-injection and a blocking
  completion-audit gate before reaching VERIFIED (ported from claude-goal)
* spec share URLs replaced with short pilot-shell.com/s/<id> backed by a
  Vercel Edge endpoint on Upstash Redis (non-spoofable per-IP rate limiting,
  base64url validation, atomic INCR+EXPIRE, NX collision retry, runtime
  feedback-payload guard); legacy fragment URLs still decode; dead local
  paste service removed; new vitest harness in docs/site with 24 tests

Author: maxritter