docs: redesign AGENTS.md with concise project guardrails

[pepakriz]

Motivation

• Make the repository-level AGENTS guidance short, unambiguous and focused only on non-negotiable project rules.
• Provide clear architecture guardrails for the local-first Evolu data model and shared UI contracts so automated agents avoid high-risk mistakes.
• Move advanced workflows and lengthy playbooks into skills so AGENTS remains easy to scan and enforce.

Description

• Rewrote AGENTS.md into a structured, project-scoped document with explicit sections for scope/priorities, architecture guardrails, Evolu write rules, error/result conventions, i18n/UI rules, safe change workflow, and a high-risk files list.
• Preserved and clarified existing conventions for Evolu writes (evolu.update/upsert/insert), typed errors (defineError plus create...Error naming), and Result usage (ok/err).
• Added directive that nested AGENTS.md files override at deeper directories and that direct user/developer/system instructions take precedence.
• Stated that detailed step-by-step procedures belong in skills and kept AGENTS limited to essential, non-negotiable rules.

Testing

• No automated tests were executed for this documentation-only change.

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Codex Task