github swarm bao: address argus review on #4892

- docs/web-ui/README.md: drop the removed Credentials tile from the
  H0M3 hub list.
- api-error.ts, hive-warn.js: rewrite comments pointing at
  dashboard/src/credentials.js and credentials.html, now deleted, to
  state what the code does instead.
- swarm-secret-client/src/github.rs: correct the Credential.value doc
  to the actual read command (bao kv get -format=json | jq
  .data.data.value), keeping the load-bearing-field-name point.
- github-token.nix, agent-github-bao.nix, LinkGithubAccountForm.tsx:
  restate added comments as current behaviour instead of changelog
  wording ("has always had", "holds the token now").

Refs #4347

frontend/packages/swarm-ui/src/pages/LinkGithubAccountForm.tsx

@@ -49,7 +49,7 @@ export function LinkGithubAccountForm({
         return;
       }
       setResult({ status: "done" });
-      // The store holds the token now; nothing here needs it.
+      // The store holds the token; nothing here needs it.
       setToken("");
     } catch (err) {
       setResult({ status: "error", problem: { detail: String(err) } });