feat(mosaic): drill-down main menu + provider-first flow + quick start

Replace the linear 12-stage wizard interrogation with a menu-driven
architecture. Key changes:

- Main menu with drill-down sections: Quick Start, Providers, Agent
  Identity, Skills, Gateway, Advanced, Finish & Apply
- Quick Start path: 3 questions (API key, admin email, password) with
  sensible defaults for everything else. Target: under 90 seconds.
- Provider-first flow: LLM API key collection moves to the front of
  the wizard, with auto-detection of Anthropic vs OpenAI from key prefix
- Deterministic intent/naming: user picks an intent category (general,
  software-dev, devops, research, content, custom) and gets a thematic
  agent name proposal (Mosaic, Forge, Sentinel, Atlas, Muse)
- Headless backward compat: MOSAIC_ASSUME_YES=1 + env vars still work
  unchanged, with new MOSAIC_AGENT_NAME and MOSAIC_AGENT_INTENT vars
- Version bump: 0.0.26 -> 0.0.27

Closes #438

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

packages/mosaic/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@mosaicstack/mosaic",
-  "version": "0.0.26",
+  "version": "0.0.27",
   "repository": {
     "type": "git",
     "url": "https://git.mosaicstack.dev/mosaicstack/mosaic-stack.git",