[User Story] Create developer onboarding guide for ghost-stack

[noahwhite]

Story Summary

As a developer working in ghost-stack, I want a comprehensive onboarding guide, so that I can quickly understand the project's workflow, conventions, and tooling without having to piece together information from multiple scattered documents.

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✅ Acceptance Criteria

• docs/developer-guide.md exists and is merged to main
• Guide covers: branch naming, PR workflow, commit signing requirements, CI/CD pipeline overview, and how to use Claude Code review (@claude comments)
• Guide links to existing runbooks and docs rather than duplicating them
• Guide is accurate and reflects the current state of the project

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📝 Additional Context

• Existing docs to reference/link: git-workflow-mvp.md, automated-deployment-workflow.md, branch-protection-rules.md, secrets-management.md, runbooks
• Should include the Claude Code review workflow (GHO-114): comment @claude on any PR to trigger an AI review
• Should replace/supersede the outdated git-workflow-mvp.md which predates CI/CD and refers to an old repo name
• Audience: future collaborators or Noah returning after a long break

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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
• Peer-reviewed
• Docs updated (if applicable)
• No critical bugs/regressions

[linear]

GHO-115