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/module-eval/agent-github-bao.nix

@@ -72,8 +72,8 @@ let
         && lib.hasInfix ".data.data.value | strings" s;
     }
     {
-      # The agent user owns its state dir (./user.nix), and the file keeps the
-      # `0600` it has always had.
+      # The agent user owns its state dir (./user.nix), and the file keeps
+      # its `0600` mode.
       name = "the fetch runs as the agent, with its own store identity";
       ok =
         let