ci: add .mjs to check-comment-blocks.sh + check-issue-refs.sh's scope

mara: "add mjs to the lint and fix it in this pr". Both lints scope their
git ls-files scan to a fixed extension list that never included .mjs, so
all three frontend build.mjs files (swarm-ui, dashboard, agent) were
completely invisible to both -- not merely under the limits, outside the
scan entirely.

check-comment-blocks.sh: added '*.mjs' to the ls-files glob and to the
mode_of regex (same slash-comment treatment .js/.ts/.tsx already get --
.mjs is the same comment syntax, argus's exact proposed fix). check-issue-refs.sh:
added '*.mjs' to its glob list, no regex change needed.

Fixing the scope immediately surfaced real, pre-existing violations in
the other two build.mjs files (swarm-ui's own is already handled by this
PR's earlier commits):
- dashboard/build.mjs: 49-line comment block (trimmed to 15 -- redundant
  per-file output-layout table cut, the code below already names every
  output path) plus three bare tracker tags (#448 x2, #453) in the
  stream-worker comment, reworded to prose
- agent/build.mjs: two bare tracker tags (hyperhive#3685 x2), reworded

No logic changes anywhere -- comment-only edits plus the two lint
scripts. Closes #4300 and (independently filed, consolidated into that
one) #4299.

frontend/packages/agent/build.mjs

@@ -1,7 +1,7 @@
 // esbuild build for @hive/agent. Output layout (`dist/`):
 //
-//   dist/index.html            served at GET / (the Preact rewrite,
-//                               hyperhive#3685 — app.js is deleted)
+//   dist/index.html            served at GET / (the Preact rewrite —
+//                               the old vanilla-JS app.js is deleted)
 //   dist/stats.html            served at GET /stats
 //   dist/screen.html           served at GET /screen
 //   dist/static/stats.js       served at /static/stats.js (pulls in
@@ -50,7 +50,7 @@ await build({
   loader: { ".css": "text" },
 });
 
-// The main agent page (hyperhive#3685's Preact rewrite — app.js,
+// The main agent page (the Preact rewrite — the old vanilla-JS app.js,
 // which used to share this build step, is deleted). A separate bundle
 // from stats.js above rather than folded in: different loader needs
 // (`.css: 'css'`, real stylesheets a component imports, vs. the raw-