fix(opencode-plugin): report disk-cache age in the stale-fallback warning (#13426)

Merged after boarding with #13185 into one worktree cut from `release/v3.8.51` (both verified as ancestors of the combined HEAD before validating).

**Evidence**
- Full `@omniroute/opencode-plugin` suite — not just this PR's file: **373/373 pass** across all 26 test files.
- Gates on the combined tree: `check-changelog-integrity` PASS, `check-complexity` PASS (2842 vs baseline 3218), `check-cognitive-complexity` PASS (1284 vs 1437), `typecheck:core` PASS.
- `check-file-size`: no violation attributable to this PR.

**One note for the record.** On a first run the suite reported `scaffold: built ESM default export resolves with the v1 plugin shape` failing with `ERR_MODULE_NOT_FOUND … dist/index.js`. That was a fresh worktree without the plugin built, not a defect in this PR — after `npm run build` in the workspace, all 373 pass. Flagging it because that failure reads exactly like a PR bug and could easily be misattributed to you on a future run.

Thanks, @RaviTharuma — putting the snapshot age in the warning turns "using stale disk cache" from a fact into something an operator can act on, and you covered `snapshotAgeLabel` with both the numeric-`writtenAt` and missing-`writtenAt` cases rather than only the happy path.

@omniroute/opencode-plugin/src/index.ts

@@ -5556,9 +5556,17 @@ export function createOmniRouteConfigHook(
         if (modelsFetchThrew && wantDiskCache && !warmSnapshot) {
           const snapshot = await diskSnapshotReader(resolved.providerId, snapshotFingerprint);
           if (snapshot && snapshot.rawModels.length > 0) {
+            // Report snapshot age like the warm-startup path already does:
+            // "stale" alone reads as a transient blip, so a week-old catalog
+            // is indistinguishable from a five-minute-old one.
+            const snapshotAge = snapshot.writtenAt;
+            const snapshotAgeLabel =
+              typeof snapshotAge === "number"
+                ? `${Math.round((Date.now() - snapshotAge) / 3_600_000)}h`
+                : "unknown";
             logAt(
               "warn",
-              `config shim: /v1/models unreachable; using stale disk cache (${snapshot.rawModels.length} models)`
+              `config shim: /v1/models unreachable; using stale disk cache (${snapshot.rawModels.length} models, age ${snapshotAgeLabel})`
             );
             localRawModels = snapshot.rawModels;
             localRawCombos = snapshot.rawCombos;