feat(sync): bootstrap prompt variations from LaunchDarkly

[ctawiah]

Summary

Adds the first testable LaunchDarkly-to-local flow for Phase 1 prompt variations.

• Lists projects and AI Configs through paginated API requests.
• Lets users select a project, AI Config, and one or more prompt variations interactively.
• Bootstraps .launchdarkly when it is missing and supports adding variations with ldcli sync prompt --add.
• Marks existing local variations as unavailable so bootstrap never overwrites them.
• Reuses the atomic local variation store and works with Git or non-Git workspaces.
• Includes no tool, skill, source metadata file, or source-selection behavior.

Verification

• go test ./internal/sync/api ./internal/sync/bootstrap ./internal/sync/local ./cmd/sync -count=1
• go test ./cmd -count=1
• Remaining commit hook passed.