feat(sync): compile local prompt variations

[ctawiah]

Summary

Compiles prompt variation files across projects in a .launchdarkly workspace into deterministic sync input. This layer builds on the variation parser in the preceding PR.

• Walks each project configuration directory and ignores unsupported files.
• Sorts variations by project and lookup key for stable request construction.
• Includes the failing file path in parse errors and reports a missing .launchdarkly directory clearly.

Verification

• go test ./internal/sync/... -count=1
• Focused lint for the changed sync packages

[ctawiah]

Consolidated into #788 so prompt variation parsing and compilation can be reviewed as one coherent local-loading change.