frontend: hive-warn — replace severity+pulse with info/warning/error levels

mara, PR review: "i dont like that they still have different styling.
if anything, there should be distinction between info,warning,error
(semantics). all warnings should be styled identically."

Replaces the severity ('amber'|'red') + standalone pulse boolean with
a single level ('info'|'warning'|'error') attribute -- a fixed
three-tier ladder instead of an open combination. Colours reuse
theme.css's own already-documented semantics rather than inventing new
ones: --cyan is already "info accents", --amber already "warnings",
--red already "errors, fail state". Pulse is now baked into `error`
specifically rather than a separate opt-in knob, since the one call
site that wanted attention-grabbing (an active incident) is also the
one that's semantically error -- tying the two together removes a
combination that shouldn't exist independently of the tier.

Reclassified the three call sites explicitly (no implicit default
relied on): credentials.html's GitHub PAT advisory and core.js's K3PT
ST4T3 caveat are both `level="warning"` (same as before, and now
identical to each other by construction, not by coincidence);
swarm.js's port-collision banner is `level="error"` (an active
incident needing operator action now, not a standing caveat).

npm run build clean both packages, grepped for leftover
severity/pulse references (only prose mentions describing what this
replaces).

frontend/packages/dashboard/src/core.js

@@ -50,7 +50,7 @@ function renderTombstones(s) {
   // Wording is deliberately about what the list *is* rather than what it
   // isn't: nothing records a destroy, so "container absent" is the only thing
   // the backend can actually tell.
-  const warn = el('hive-warn', {});
+  const warn = el('hive-warn', { level: 'warning' });
   warn.append(
     el('strong', {}, 'shows every agent whose container is absent'),
     ' — not only destroyed ones. An agent part-way through being spawned ' +