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

swarm-secret-client/src/github.rs

@@ -35,8 +35,9 @@ pub fn account_path(agent: &str) -> Result<String, Error> {
 #[derive(Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub struct Credential {
     /// The token. `github-token.nix` reads the store with
-    /// `bao kv get -field=value`, so this name is load-bearing for a reader
-    /// this crate does not control.
+    /// `bao kv get -format=json` and pulls `.data.data.value` out with
+    /// `jq`, so this name is load-bearing for a reader this crate does not
+    /// control.
     pub value: String,
 }