docs: add CLAUDE.md for AI assistant onboarding

[allerion-systems]

What

Adds CLAUDE.md to give AI coding assistants accurate context before they touch this repo.

Why

This repo is the org-wide .github community-health repo for allerion-systems. It currently contains only profile/README.md (the org homepage README) and one init commit — nothing else. That's easy for an agent to misjudge on a quick glance, and files added here later (CODEOWNERS, SECURITY.md, workflow-templates, issue/PR templates) become silent org-wide fallback defaults for any repo in the org that lacks its own copy — a non-obvious GitHub behavior with a wide blast radius.

What the doc covers

• What this repo is (org community-health defaults) and its current, verified contents
• A file-by-file inventory (currently just profile/README.md)
• How GitHub propagates files from this repo as org-wide fallbacks, and why that matters before editing
• That no workflow-template/issue-template conventions exist yet, and to follow GitHub's documented structure if any are added
• What an agent must not casually change here (org homepage branding, and any future CODEOWNERS/SECURITY/workflow-template files) given the org-wide blast radius

No other files were touched.

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