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

nix/agent-modules/github-token.nix

@@ -74,7 +74,7 @@ in
         Group = agentName;
         RuntimeDirectory = runtimeDir;
         RuntimeDirectoryMode = "0700";
-        # `0600`, the mode `github-token` has always had.
+        # `0600`, the mode `github-token` has.
         UMask = "0077";
         LoadCredential = [
           certCredential