Polish onboarding and Agent Packs navigation UX

[snissn]

Summary

Resolves #22.

This PR completes the pre-merge UX cleanup for the Claude-compatible skill RC:

• shortens and clarifies the homepage onboarding prompt preview
• simplifies /agents so cards lead to one pack page instead of forcing a format choice immediately
• moves version and format selection onto a unified pack detail page
• keeps secondary actions available, but demoted behind More

What changed

Homepage onboarding

• rewrote the compact onboarding prompt preview to be shorter and outcome-first
• kept the full long-form copy action intact
• clarified that the copy button uses the full prompt, not only the preview

Agent Packs registry

• changed pack cards to use one primary CTA: Open Pack
• demoted direct canonical/Claude/provider actions into a smaller More menu
• turned file path pills into metadata instead of primary navigation
• removed older-version navigation from the card surface

Unified pack page

• added /agents/pack
• the pack page owns:
  • version selection
  • format selection
  • lower-priority secondary actions
• kept existing direct /agents/spec and /agents/skill pages for compatibility, but retargeted them back to the unified pack view via Pack Overview

Validation

• cd website && npm run build

Notes

• Logged-in Claude Code validation remains deferred in GA follow-up: validate Claude-compatible skills in logged-in Claude Code #21.
• This PR is intended to land before the final RC merge surface in RC: Claude-compatible skill distribution rollout #20.

[snissn]

Pre-merge remediation: fixed the Markdown Pack CTA so Copy Pack Text now copies the actual pack content instead of a prompt wrapper. Revalidated with cd website && npm run build.