docs: add CLAUDE.md with rain.flare build invariants (#74)

[thedavidmeister]

Summary

• Adds CLAUDE.md documenting the load-bearing build invariants: which files are generated, how to regenerate them, nix-shell-only tools, soldeer dependency management, and auto-publish on push to main.
• .soldeerignore already lists CLAUDE.md (line 15), so the file is excluded from soldeer package distribution.

Closes #74

Test plan

• Docs-only change; no code or artifact impact.
• CLAUDE.md is in .soldeerignore, so it won't appear in published soldeer packages.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Documentation
  • Added contributor guidance covering build requirements, regeneration of generated artifacts, dependency installation, and publishing behavior.
• Chores
  • Included the new documentation file in the project’s existing reuse/license coverage rules.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: e90f70e7-ff46-4cc6-ae7d-bdafc3efc3a3

📥 Commits

Reviewing files that changed from the base of the PR and between cb20d03 and 4dd9264.

📒 Files selected for processing (2)

• CLAUDE.md
• REUSE.toml

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Walkthrough

Adds a CLAUDE.md file at the repo root documenting build invariants: artifact regeneration steps, required nix develop tooling, Soldeer dependency installation, and automatic publishing on push to main. Updates REUSE.toml to include CLAUDE.md in its annotation paths.

CLAUDE.md and REUSE registration

Layer / File(s)	Summary
Build invariants doc CLAUDE.md, REUSE.toml	CLAUDE.md documents regeneration of committed artifacts (meta/*.rain.meta, src/generated/FlareFtsoWords.pointers.sol), mandatory nix develop tooling, Soldeer install, and auto-publish on main. REUSE.toml adds CLAUDE.md to the annotated paths.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the new CLAUDE.md docs for build invariants.
Linked Issues check	✅ Passed	The PR adds the requested root CLAUDE.md with the key build and publish invariants.
Out of Scope Changes check	✅ Passed	The only code change beyond the doc file is the related REUSE.toml annotation update.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/issue-74-claude-md

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[thedavidmeister]

human-approved; merging