chore: fix lint script for Next 16, update Open Plugins links, document detected manifests and commands

[deanrie]

Summary

Housekeeping, no behaviour change:

• lint script. apps/cursor/package.json still has "lint": "next lint", which Next.js 16 removed, so bun run lint fails while AGENTS.md lists it as a standard command. CI already uses Biome (bunx biome ci .); the script now runs biome check . so local and CI agree.
• Open Plugins links. open-plugins.com redirects (308) to agent-plugins.org, and /plugin-builders/specification redirects twice to /specification. Updated the README, the submission form, the /plugins/new page and the parser's error message to the current host. nuqs.47ng.com likewise moved to nuqs.dev.
• README auto-detect table. The parser also accepts mcp.json (not only .mcp.json), detects commands/*.md, reads the manifest from .plugin/, .cursor-plugin/ or .claude-plugin/, and scans monorepo sub-plugins listed in .cursor-plugin/marketplace.json. The table and a short note now say so, which should cut down on "my repo wasn't detected" questions.

Verification

• bun x biome ci . and bun run lint (now Biome) both pass; bun run typecheck unchanged.
• Redirect targets checked with curl -L today: all resolve 200 at the new hosts.

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