feat(phase-4): WS2 + WS3 — Developer Portal (Next.js 14) and CLI tool (sentryagent)

WS2: Developer Portal (portal/)
- Standalone Next.js 14 + Tailwind CSS app — independent deployment
- Home page: hero, feature grid, CTA to /get-started
- /pricing: free tier limits table (10 agents, 1k calls/day) + paid tier CTA
- /sdks: all 4 SDKs (Node.js, Python, Go, Java) with install + code examples
- /api-explorer: Swagger UI from NEXT_PUBLIC_API_URL/openapi.json, persistAuthorization
- /get-started: 4-step wizard (setup → register agent → credentials → SDK snippet)
- Shared Nav component with active-link highlighting
- Build: 8/8 static pages, zero TypeScript errors

WS3: CLI Tool (cli/ — npm package: sentryagent)
- configure, register-agent, list-agents, issue-token, rotate-credentials, tail-audit-log
- Auto OAuth2 token fetch + 30s-buffer cache via client_credentials flow
- chalk-formatted table output, confirmation prompts, bounded audit log dedup
- bash + zsh shell completion scripts
- README with installation, all commands, and completion setup
- Build: tsc clean, node dist/index.js --help verified

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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