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 known equivalent model defaults: absent versus empty parameters and custom maps.
• Resolve variation model references before generic reconciliation, without adding variation-specific state to PlannedResource.
• Treat an omitted custom model version as “follow latest” at sync time; explicit versions remain pinned.
• Keep authored files unchanged while planning and applying their concrete effective state.
• Re-read every successful server mutation before advancing the manifest.
• Keep unversioned model configs unchanged and preserve meaningful nested model differences.

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 ./...
• Repeated create, update, and matching-server convergence acceptance tests
• git diff --check