feat(sync): preview prompt variation plans

[ctawiah]

Summary

Adds a testable ldcli sync prompt preview flow on top of the variation planning client. The command analyzes changes without creating a durable plan or applying updates.

• Resolves Git or local source identity automatically with no source-selection flag.
• Compiles local prompt variations and sends dry-run plan requests by project.
• Displays project, variation, status, sync direction, proposed action, diff, and resource errors in JSON, plaintext, or Markdown output.
• Registers the sync command and adds analytics instrumentation consistent with other top-level commands.

Verification

• go test ./cmd/sync -count=1
• Focused root command and usage-template tests
• Focused lint for ./cmd/sync

[ctawiah]

Consolidated with the planning client in #791 so the preview flow can be reviewed end to end.