feat: add global CLAUDE.md for TDD

[scc-tw]

Summary

Introduce a global CLAUDE.md at the repository root to standardise how we collaborate with LLM pair‑programmers (e.g. Claude) using Test‑Driven Development (TDD) and Kent Beck’s Tidy First

Context & Rationale

• Proven template – Kent Beck’s BPlusTree3 project ships a concise CLAUDE.md that drives an automated Red → Green → Refactor workflow for every change.
• Enforces canonical TDD cycle – centralising the rules keeps every contributor (human or AI) on the same page about writing the failing test first, making it pass, then refactoring.
• Separates structure from behaviour – Tidy First insists we commit structural tidies separately from behavioural changes, reducing review burden and merge conflicts.
• Small, frequent commits – numerous case‑studies show that tight Red‑Green loops plus micro‑tidies accelerate delivery and cut rollback cost.
• Scales across monorepo – a single root‑level file avoids duplicated instructions inside each package or service.

Proposal

1. Copy & adapt rust/docs/CLAUDE.md (§Role, Core Principles, Commit Discipline). Keep wording but replace examples with our language‑specific conventions.
2. Place the new file at /<repo‑root>/CLAUDE.md.
3. Amend CONTRIBUTING.md to reference the file.
4. Add a CI guard (simple grep or markdown‑lint rule) that fails if the file is removed or edited without running make test.

Acceptance Criteria

• CLAUDE.md exists in the repository root and renders without lint warnings.
• CI pipeline fails when a PR violates any Commit Discipline bullet (e.g., structural + behavioural change in same commit).
• A link to CLAUDE.md appears in Contributing guidelines.
• At least one green‑to‑red‑to‑refactor demonstration PR merged using the new workflow.

Tasks

• Draft CLAUDE.md (copy → adapt wording → PR)
• Update CONTRIBUTING.md with a “Working with Claude” subsection
• Implement CI check (bash / pre‑commit)
• Showcase example PR proving the loop
• Announce the change in #dev‑announce