feat(sync): bootstrap prompt variations from LaunchDarkly

[ctawiah]

Context

This PR adds the first user-facing LaunchDarkly-to-local workflow for Phase 1 prompt variation sync. It is intended for initial setup when .launchdarkly does not exist and for explicitly adding more server variations later with ldcli sync prompt --add.

The interactive flow discovers projects, AI Configs, and variations through the API, then delegates file creation to the atomic local store from the preceding PR. Existing local variations are unavailable for selection, so bootstrap and add cannot overwrite local work. This PR does not update existing files or apply local changes back to LaunchDarkly.

Summary

• Lists projects and AI Configs through paginated API requests.
• Lets users select a project, AI Config, and one or more prompt variations interactively.
• Writes only sync-owned fields and leaves the remote variation comment server-owned.
• Bootstraps .launchdarkly when it is missing and supports adding variations with ldcli sync prompt --add.
• Keeps bootstrap and add selections read-only with --dry-run, printing each target path and exact file content without creating or changing local files.
• Clearly separates each dry-run file preview so paths and content are easy to distinguish.
• 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/... ./cmd/sync -count=1