fix(core): skip models.dev refresh event when the catalog is unchanged

[thdxr]

Problem

The models.dev service repeats refresh() every 5 minutes (Schedule.spaced(ttl)), and because the repeat interval equals the freshness TTL, every cycle fetched a new body, rewrote the multi-MB KV entry, invalidated the memoized catalog, and published ModelsDev.Event.Refreshed — even when api.json was byte-identical.

That triggered the full downstream chain on every cycle with no content change:

1. Refreshed → the opencode.models.dev plugin re-runs its integration/catalog transforms (ctx.catalog.reload())
2. Catalog.finalize publishes catalog.updated unconditionally (no diffing)
3. Every listener churns: the TUI subscribes to catalog.updated, and the event logger writes it

Fix

Persist a sha256 digest of the raw response body alongside the KV cache entry. refresh() still fetches each cycle (same cadence as before), but when the new body is byte-identical to the cached one it now skips:

• the cache rewrite (saves a multi-MB KV write per cycle — notable given Durable Object SQLite's 2 MB value cap noted in the existing comment)
• the catalog invalidation
• the Refreshed publish

refresh(true) (explicit force) bypasses both the freshness check and the digest skip, preserving its "reload now" semantics for callers like tests/CLI.

Legacy cache entries written before this change have no digest; they decode fine via the optional schema field and publish once before self-correcting.

Notes

• The server does serve an ETag (models.opencode.ai/api.json, Cloudflare), so conditional If-None-Match requests could additionally save bandwidth. That is left out of scope here: 304 handling would need special-casing around HttpClient.filterStatusOk + retryTransient (304 is not an ok status and would be retried). The digest comparison achieves the behavioral goal with no protocol complexity.
• Only subscriber of Refreshed is the models-dev plugin, so no other call sites are affected.

Testing

• Extended refresh(false) fetches when stale to assert one Refreshed event is published on a real content change
• New: unchanged byte-identical body → HTTP still hit once, but no Refreshed event and cache entry untouched
• New: legacy entry without digest → republishes exactly once and persists the digest
• All 15 tests in packages/core/test/models.test.ts pass; bun typecheck clean in packages/core