Add Agentic Commerce Hackathon page

Description

The pitch.

## Summary
Adds a page promoting the Agentic Commerce Hackathon (Devfolio, July 31 - Aug 2 2026), where Project NANDA is a partner sponsor.

- New route at `/agentic-commerce`: hero (flyer image, links to the Devfolio apply page), prize pool breakdown, what-to-build, eligibility + participation steps, judges & speakers, closing CTA.
- New nav entry under the "NandaHacks" dropdown pointing to the new page.
- Config data lives in `lib/agentic-commerce-event.ts`, single source of truth, same convention as the existing `lib/hackathon-event.ts`.
- No changes to any existing hackathon content (NandaHack x HCLTech banner/FAQ/phases) or any other route.

## Test plan
- [x] `npm run build` — clean, `/agentic-commerce` generated as a static route
- [x] `npm run lint` — no new errors introduced (12 pre-existing errors elsewhere, unrelated to this change)
- [x] Manual dev-server check of the page and nav link
- [x] Verified the Devfolio outbound link (200), the hero image asset, and the nav link all resolve correctly

Leaving this open, unmerged, for a maintainer to review.