docs: rewrite root README — narrative, comparison, honesty, ADR links

[EmersonBraun]

Summary

Story-driven rewrite of the root README. Closes #224 and #16.

The previous README was decent but generic ('complete toolkit'). This version positions AgentsKit as opinionated, contracted, honest — and routes readers immediately to the Manifesto, Origin, ADRs, and agentskit.io.

What changed

New

• Opinionated tagline — 'The agent toolkit JavaScript actually deserves'
• 'Why this exists' section calling out the actual gaps in Vercel AI SDK / LangChain.js / MCP / assistant-ui — and linking to ORIGIN.md and MANIFESTO.md for the long version
• Before / After section — cobbled multi-library stack vs two-import AgentsKit equivalent
• Honest 'When you should NOT use AgentsKit' section directing readers to:
  • The OpenAI SDK directly for single-call use cases
  • Vercel AI SDK for chat-only products
  • Python frameworks
  • Mature observability platforms (LangSmith/Arize/Helicone)
  • A v1.0.0 wait if pre-1.0 risk is a blocker
• Architecture and contracts section linking all six ADRs
• Status section linking Phase 0 PRD [Master PRD] Phase 0 — Foundation Hardening (antes da Fase 1) #211 and Master PRD [Master PRD] AgentsKit Roadmap — 4 phases, 97 user stories #113
• For AI agents callout — first-class consumers of our docs
• agentskit.io as canonical docs URL

Tightened

• Single 10-line quickstart instead of three competing ones
• Comparison table now includes formal-contracts and edge-readiness rows
• Ecosystem table now shows stability tier per package

Preserved

• Multi-agent delegation example
• Provider-swap-in-one-line demonstration
• Terminal chat callout

Honest about dependencies

This PR references files that land in other open PRs:

• MANIFESTO.md, ORIGIN.md, RFC issue template — in foundation: manifesto, origin, governance templates, and Phase 0 planning docs #265
• ADR 0001-0006 — in docs(adr): 0001 — Adapter contract (v1) #267 docs(adr): 0002 — Tool contract (v1) #270 docs(adr): 0003 — Memory contract (v1) #271 docs(adr): 0004 — Retriever contract (v1) #272 docs(adr): 0005 — Skill contract (v1) #273 docs(adr): 0006 — Runtime contract (v1) [closes #214] #274
• ADR 0007 (Fumadocs) — in docs: migrate to Fumadocs (44 pages, 12 sections, ADR 0007) #277

Recommended merge order: #265 → #267..#274 → #277 → this PR. Links resolve correctly once the referenced files are on main.

Test plan

• Markdown renders cleanly on GitHub
• All internal anchors resolve (Manifesto, Origin, ADRs, packages/*)
• Code blocks compile in pnpm lint (TypeScript-only, paste-test)
• Reviewer: read aloud — does the tone feel earned or pushy?
• Reviewer: spot-check 'When you should NOT use' section for fairness
• Reviewer: comparison table accurate vs current state of competitors?

Refs #224 #16 #211