feat: add gradient startup screen and remove old OPEN box logo

Adds a new startup screen with filled-block text logo and sunset
gradient, printed to stdout before the Ink UI loads. Removes the
old OPEN box logo from the chat UI since the new screen replaces it.

Changes:
- src/components/StartupScreen.ts (NEW) — gradient OPEN CLAUDE logo
  with provider info box (Provider, Model, Endpoint). Auto-detects
  active provider from env vars (OpenAI, Gemini, DeepSeek, Ollama,
  Groq, Mistral, Azure, LM Studio, Anthropic). Skipped in CI and
  non-TTY environments.
- src/entrypoints/cli.tsx — calls printStartupScreen() at startup
  before Ink renders
- src/components/Messages.tsx — removes <LogoV2 /> from LogoHeader
  so the old OPEN box logo no longer appears in the chat UI

Addresses #55.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/entrypoints/cli.tsx

@@ -100,6 +100,10 @@ async function main(): Promise<void> {
 
   validateProviderEnvOrExit()
 
+  // Print the gradient startup screen before the Ink UI loads
+  const { printStartupScreen } = await import('../components/StartupScreen.js')
+  printStartupScreen()
+
   // For all other paths, load the startup profiler
   const {
     profileCheckpoint