feat: streamline first-run sticker setup

[9Ashwin]

What changed

• Keep the Agent Skill installation mandatory in both platform installers; remove the opt-out path.
• Add --pack curated|all and --source to initialize a selected pack during installation, while keeping the default image download-free.
• Support STICKER_PACK_SOURCE for a persistent local or HTTPS pack source.
• Mark a successful first-run search with data.setup_required: true so an Agent can initialize the curated pack without confusing an empty library with an empty match.
• Align README, Skill, PRD, SPEC, and issue acceptance with the updated flow.

Validation

• GOTOOLCHAIN=auto make quality
• GOTOOLCHAIN=auto go test -race ./...
• GOTOOLCHAIN=auto make e2e-agent
• bash -n scripts/install.sh
• git diff --check