[User Story] Create rclone sysext package

[noahwhite]

Story Summary

As a platform engineer, I want rclone packaged as a systemd-sysext image, so that I can use it for backup operations on Flatcar Container Linux.

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✅ Acceptance Criteria

• New repository noahwhite/rclone-sysext-build created
• Dockerfile and build script create valid sysext image
• GitHub Actions workflow builds, signs, and uploads to R2
• GPG signature verification enabled (same key as Alloy)
• SHA256SUMS manifest maintained
• Automated PR creation for ghost-stack when new version is built
• check-new-releases.yml detects upstream rclone releases

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📝 Additional Context

• Design: Follow the same pattern as alloy-sysext-build
• The sysext provides /usr/local/bin/rclone
• No systemd service included (rclone is CLI tool, not daemon)
• Uses existing GPG signing key from Bitwarden

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📦 Definition of Ready

• Acceptance criteria defined
• No unresolved external dependencies
• Story is estimated (3 points)
• Team has necessary skills and access
• Priority is clear
• Business value understood

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✅ Definition of Done

• All acceptance criteria met
• Repository created with all workflows
• Initial build triggered and successful
• PR created in ghost-stack with sysext configuration
• Documentation in CLAUDE.md

[linear]

GHO-62