Improve create-mountaineer-app first run onboarding

[pradhankukiran]

Summary

• improve first run experience so missing database tables no longer crash the default home render; instead return setup guidance to the frontend and disable mutation actions until setup is complete
• expose the existing Iceaxe-backed migrate click group as an installable project script in generated pyproject.toml
• update generated project README with explicit local Postgres + migration-first setup flow (createdb remains available as fallback)
• add optional onboarding prompt to generate agentic config files and include scaffold-specific templates for AGENTS.md, Claude.md, and .cursorrules
• update CMA docs prompt example and add/adjust builder tests for hidden file allowlist, migrate script generation, and agentic file generation

Testing

• uv run pytest create_mountaineer_app/__tests__/test_builder.py create_mountaineer_app/__tests__/test_generation.py create_mountaineer_app/__tests__/test_cli.py

Closes #215