learnings: surface persist failures as warnings, not debug

`persistLearnings` only emitted `log.info("» learnings updated")` on
success; every failure path (non-2xx, fetch throw, 10s timeout) was
`log.debug`, which is hidden unless `ACTIONS_RUNNER_DEBUG=true`. Survey
of recent runs caught at least one case where the agent definitively
edited the tmpfile but no DB row was written and no warning surfaced.

Promote both failure paths to `log.warning` so dropped agent work is
visible in CI logs. The unchanged-from-seed short-circuit stays at
debug — that's a genuine no-op.

main.ts

@@ -471,12 +471,16 @@ async function persistLearnings(ctx: ToolContext): Promise<void> {
     });
     if (!response.ok) {
       const error = await response.text().catch(() => "(no body)");
-      log.debug(`learnings persist failed (${response.status}): ${error}`);
+      // promoted from debug → warning: this path means the agent edited the
+      // file (we already short-circuited the unchanged-from-seed case above)
+      // but the PATCH dropped it on the floor. silently losing real work is
+      // worse than the noise of a CI warning.
+      log.warning(`learnings persist failed (${response.status}): ${error}`);
       return;
     }
     log.info("» learnings updated");
   } catch (err) {
-    log.debug(`learnings persist failed: ${err instanceof Error ? err.message : String(err)}`);
+    log.warning(`learnings persist failed: ${err instanceof Error ? err.message : String(err)}`);
   }
 }