docs: improve README with full CLI reference, --platform docs, and Troubleshooting

[AngryJay91]

Closes #16

Changes

• CLI Reference now covers all 5 commands: init, hydrate, deploy, migrate, doctor (previously missing deploy and migrate)
• Per-command detail sections with usage examples, prerequisites, and behavior notes
• --platform option documented with a table of supported adapters (claude-code, codex, gemini) and examples
• Troubleshooting section added covering 9 common issues:
  • Existing structure detected on init
  • Templates not updating
  • deploy fails (wrangler.toml missing, not authenticated)
  • migrate duplicate column warnings
  • MCP connection not working
  • Oscar/slash commands missing
  • spec-site startup errors
  • doctor failures

Constraints respected

• Only README.md modified
• No TypeScript/JavaScript code touched
• project.yaml.example checked — all fields present, no additions needed