feat(init): add template system for project-type-aware setup

[MrFlounder]

Why

crab init previously generated a minimal config and relied on crab config scan to detect ports after the fact. For known project types like promptfoo-cloud, this left users with an incomplete config missing critical fields (port_spacing, install_env, admin-app/.env, API_PORT/PORT refs) -- the same gaps identified in PR #3185.

What

• Added a template system with 4 new functions: template_promptfoo_cloud(), detect_template(), apply_template(), show_templates()
• Rewrote show_init() to support:
  • --template <name> / -t <name> flag (with pf alias for promptfoo-cloud)
  • --list-templates flag
  • Auto-detection of project type from repo contents (checks .gitmodules for promptfoo submodule + server/.env.example)
• Updated command routing to pass args through: show_init "${@:2}"
• Updated help text to reflect new flags
• Also updated Slack token config to prefer CRAB_SLACK_BOT_TOKEN env var over config file entry

The promptfoo-cloud template includes:

• port_spacing: 10
• install_env: PROMPTFOO_NODE_MODULES_CACHED=true
• admin-app/.env in env_sync.files
• API_PORT and PORT refs in env sync

How to Test

1. crab init --list-templates -- should display the promptfoo-cloud template
2. crab init -t promptfoo-cloud -- should apply the template and generate a full config
3. crab init -t pf -- alias should also work
4. Clone a promptfoo-cloud repo and run crab init -- should auto-detect and offer to apply the template
5. Run crab init in a non-matching repo -- should fall through to the minimal 2-question setup