v0.3.0 — blank-by-default wizard, live run console, model picker, plugin pre-flight

Public-ready polish: the wizard no longer auto-fills with demo data, you
can pick which Claude model runs DisC, and the "Run it for me" pipeline
streams progress live with an 8-step checklist and a working Cancel
button. Plus a Claude Code plugin pre-flight that catches "DisC isn't
installed" before you hit run, and surfaces updates when they're available.

Added

• Live DisC run console. "Run it for me" now streams the subprocess's
  stream-json output back into the wizard as it happens: an 8-step
  checklist (init → tests → green → impl → coverage …) ticks through in
  real time, the elapsed timer updates each second, and a Cancel button
  cleanly tears down the spawned claude process.
• Model picker on Step 4. Pick Sonnet 4.6 (default), Opus 4.7, or
  Haiku 4.5 next to the "Run it for me" button. The selection is passed
  to the spawned subprocess as --model <id> against a server-side
  allowlist; no other models are accepted. Doesn't affect the slash
  command you copy for interactive use.
• DisC plugin pre-flight. Before the wizard offers "Run it for me"
  it checks whether design-is-code is installed in your Claude Code
  config and which version. If missing, an inline "Install plugin"
  banner runs claude plugin install … for you. If outdated, an
  "Update plugin" banner shows the new version + changelog link and
  installs the upgrade with one click.
• "Load demo data" button in the header (next to the Connect-project
  chip). Click it to seed the same end-to-end "generate invoice"
  example that used to load automatically.
• Multi-modal input affordances on Step 2's Participants and Steps
  section heads — greyed-out text/image/voice/video icons signaling the
  natural-language / multi-modal input paths planned for a later release.
• Release runbook (RELEASE.md) — a checklist the
  AI agent follows when you say "release it."

Changed

• Wizard starts blank. Story textarea, participants list, sequence
  steps, target package, and project chip all start empty on page load.
  This makes the app usable for someone other than the author. Use the
  new "Load demo data" button to get the previous behavior.
• Step 4 layout tightened around the run-result panel — model picker,
  copy-command button, plugin pill, and run controls now sit on a single
  wrapping row instead of stacking.

Breaking

• The page no longer auto-prefills with the invoice example or
  auto-connects to /Users/mossgu/Downloads/demo. If you relied on a
  full-screen demo on page load (e.g. screencasts, screenshots), click
  "Load demo data" first or revert to v0.2.0.