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.
• Fetch the exact model config by key, which returns its latest version without listing the project catalog.
• 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

---

Note

Overview
Fixes prompt sync so equivalent LaunchDarkly API shapes and “follow latest” model configs no longer cause endless drift between local files, fingerprints, and server state.

Fingerprint normalization strips only no-op defaults on variation model maps—empty parameters and custom are treated like absent—while still fingerprinting meaningful empties such as metadata.

Follow-latest model configs: when a variation has modelConfigKey but omits modelConfigVersion, planning resolves the latest config via a new ModelConfig GET (by key, not catalog list), merges it into the in-memory payload for comparison, and tracks LocalFollowsLatestModelConfig. Server writes use the resolved version; local pulls keep modelConfigVersion: 0 on disk so the next sync can pick up newer model-config releases.

Execution/revalidation drops per-resource pre-write fingerprint checks in favor of a full plan re-read after confirmation (still re-reading server state only for ambiguous writes). Local apply/verify paths account for follow-latest when checking post-pull fingerprints.

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