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/shared/src/hive-warn/hive-warn.js

@@ -1,10 +1,9 @@
 // hive-warn.js — <hive-warn>, the shared inline warning-banner
-// component. Consolidates three independently-written instances of the
-// same thing: `.cred-warning` (credentials.html, static
-// markup), `.tombstone-warn` (core.js, JS-built), `.port-conflict`
-// (swarm.js, JS-built) — the first two differed only by an
+// component. Consolidates what were independently-written instances of
+// the same thing, among them `.tombstone-warn` (core.js, JS-built) and
+// `.port-conflict` (swarm.js, JS-built) — some differed only by an
 // undeliberate 10% vs 8% tint, the strongest evidence this was drift,
-// not three genuinely different needs.
+// not genuinely different needs.
 //
 // Purely presentational — no internal state, no lifecycle beyond
 // attaching its shadow root once. `level` ('info' | 'warning' | 'error',