docs: add OSS policy reference to CLAUDE.md

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: stackbilt-admin

CLAUDE.md

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+
+## OSS Policy
+
+This is a **public infrastructure package** governed by the Stackbilt OSS Infrastructure Package Update Policy.
+
+Rules:
+1. **Additive only** — never remove or rename public API without a major version bump
+2. **No product logic** — framework patterns and generic utilities only. If a competitor could reconstruct Stackbilt product architecture from this code, it doesn't belong here.
+3. **Strict semver** — patch for fixes, minor for new features, major for breaking changes
+4. **Tests travel with code** — every public export must have test coverage
+5. **Validate at boundaries** — all external API responses validated before returning to consumers