docs: add CLAUDE.md

[btravers]

Adds a CLAUDE.md at the repo root so future Claude Code sessions start with the
repository's commands, architecture and binding conventions instead of rediscovering them.

Contents

• Commands — the root turbo-delegating scripts, the full CI gate, and how to run a
  single test file or a single test from inside packages/entity.
• Architecture — the five source modules by what they own: entity.ts as the builder
  where every entry point funnels through construct, types.ts as the type-level
  derivation plus the Sealed construction lock, instance.ts's self-overwriting getter
  and why a plain value would build the base class, shape.ts's branded-field check. Plus
  the rule the design turns on — contracts compose the four plain ZodObjects, domain code
  composes instance.
• Binding conventions — errors-as-values via the unthrown/* oxlint rules, the
  measurement-citing comments (TS2411/2526/4020/4111, the pinned typescript and
  @orpc/zod catalog entries) as regression guards, *.test-d.ts as the home of
  compile-time guarantees, peer deps and the catalog: protocol.

Docs only — no source, config or behaviour changes.

[Copilot]

Pull request overview

Adds a repository-root CLAUDE.md to document the project’s development commands, core architecture, and binding conventions so future Claude Code sessions can start from an accurate mental model without rediscovering it.

Changes:

• Introduces CLAUDE.md describing the monorepo/tooling commands and CI “gate” sequence.
• Summarizes the package architecture (builder/types/instance/shape/add+errors) and key design rules (contracts vs instance).
• Records binding conventions (errors-as-values, measurement-as-regression-guard comments, type-level tests, peer deps, and catalog: dependency protocol).

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.