fix(sync): guarantee prompt fingerprint convergence

[ctawiah]

Context

Prompt sync uses fingerprints to decide whether local and LaunchDarkly variations still represent the same state. Equivalent API defaults could produce different fingerprints, and an omitted custom model version previously could not converge with the concrete version stored by LaunchDarkly.

What changes

• Normalize only equivalent API defaults: absent versus empty parameters and custom maps.
• Treat an omitted custom model version as “follow latest” at sync time; explicit versions remain pinned.
• Resolve that authored intent before planning, while leaving the local file unchanged.
• Revalidate the complete plan once after confirmation instead of rereading every resource before its write.
• Preserve the existing verification read for uncertain writes whose response may have been lost.

For example, this authored intent remains unchanged:

modelConfigKey: claude-prod

If the current version is 4, sync plans and applies the effective variation with modelConfigVersion: 4. A later model-config release is picked up by the next sync.

Verification

• go test ./internal/sync/...
• go test ./...
• go vet ./internal/sync/... ./cmd/sync
• Create and update convergence acceptance tests
• git diff --check