[User Story] Update CLAUDE.md with ephemeral compose documentation

[noahwhite]

Story Summary

As a platform engineer, I want to update CLAUDE.md with documentation for the ephemeral compose approach, so that future maintainers understand the architecture and secrets management.

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✅ Acceptance Criteria

• Architecture section added explaining ephemeral vs persistent components
• Secrets management section documenting .env.config vs .env.secrets split
• Upstream update workflow documented (how to sync with TryGhost/ghost-docker)
• Files to monitor for updates listed
• Common operations updated (e.g., where to find compose files)
• Migration checklist included for reference

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📝 Additional Context

• Design: Documentation should be actionable and include specific file paths
• Include the GitHub watch + manual sync workflow for upstream updates
• Reference the threat model for secrets management decisions

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📦 Definition of Ready

• Acceptance criteria defined
• No unresolved external dependencies
• Story is estimated
• Team has necessary skills and access
• Priority is clear
• Business value understood

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✅ Definition of Done

• All acceptance criteria met
• Documentation is clear and accurate
• Peer-reviewed
• No sensitive information in documentation

[linear]

GHO-53