feat: overhaul installer for one-command keyless install

Major changes to install.sh:
- Allow keyless start: stack runs without LLM key, configurable later in Settings
- Env vars override .env placeholders (fixes the precedence bug)
- Add --help, --version, --non-interactive, --seed-demo, --no-seed-demo, --fresh, --project-name flags
- TTY-aware prompts: prompt via /dev/tty if available, never block without TTY
- Print NEEDS_USER_INPUT for missing optional inputs in non-interactive mode
- Auto-generate admin password if not provided
- Fix: print login email instead of 'username: admin'
- Detect Docker permission error and print exact fix
- Warn on reinstall with existing volumes (offer --fresh)
- Demo seeding now defaults to ON

Major changes to remote-install.sh:
- Now runs install end-to-end by default (was just clone + print instructions)
- Latest release detection retries, then falls back to git ls-remote
- Fails loudly if release detection fails (no silent fallback to main)
- Pass through all flags to install.sh
- Renamed --branch to --ref for clarity

Fixes P0-1: the one-liner now performs the full install.

Co-authored-by: Venkat SF <venkatesh.sakamuri@stayflexi.com>

Author: cursoragent