[User Story] Forward-port sync-tryghost-compose guard step to main branch

[noahwhite]

Story Summary

As a platform engineer, I want the Sync TryGhost Compose Images workflow on the main branch to include the develop-branch guard step, so that the scheduled run no longer executes the full sync logic from main.

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✅ Acceptance Criteria

• sync-tryghost-compose.yml on main includes the guard step that checks GITHUB_REF_NAME and skips gracefully when not on develop
• Scheduled runs triggered from main exit cleanly with a notice, no downstream steps executed
• The workflow on main is identical to develop for the guard step logic

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📝 Additional Context

• GHO-121 added the guard step to develop via PR fix: skip sync gracefully when triggered from non-develop branch (GHO-121) #333, but main was not updated — main still has the pre-fix version of the workflow
• Scheduled workflows run from the default branch (main), so the guard never fires and the full sync logic runs nightly from main
• Fix: open a PR directly to main that forward-ports the guard step from develop
• Related: GHO-121 (original fix), PR fix: skip sync gracefully when triggered from non-develop branch (GHO-121) #333

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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
• Unit/integration tests written & passing
• Peer-reviewed
• Docs updated (if applicable)
• Verified in staging (if needed)
• No critical bugs/regressions

[linear]

GHO-125