build agents only from internal/meta; delete the hive's local meta flake

The hive no longer renders, locks or commits a meta flake. A rebuild's
MetaSync node picks the internal/meta commit and records it with the
hyperhive and config pins it holds; Prebuild, Swap and Create build from it,
and a head that does not pin the agent's current config fails the build
naming the agent. hive-priv's meta_rev is required.

Removed: meta.rs sync/lock/render, the manual-rebuild relock, the boot
sweep's hyperhive bump (MetaLock), the update-meta-inputs form and its
panel, MetaWindow, the hive's hyperhive_rev setting.

Kept: agent_set::refresh holds what sync_agents also did (topology.json, the
gateway's agent-socket map and nginx include), called from spawn, destroy,
rebuild and the migration.

The boot reconcile and the stale-start upgrade compare the agent's marker
with the hyperhive pin of internal/meta; the dashboard's deployed chip reads
the config marker a successful build writes.

frontend/packages/shared/src/forms.js

@@ -40,7 +40,7 @@ export function asyncBtn(btn, fn) {
 }
 
 // Page-level submit interceptor for `data-async` forms — the pattern every
-// dashboard/agent action button uses (meta-update, spawn, cancel, purge,
+// dashboard/agent action button uses (spawn, cancel, purge,
 // rebuild, …). Without this a `data-async` form POSTs natively and the
 // browser navigates to the bare `ok` response page. Each page that renders
 // such forms must call this once at boot. `onSuccess` runs after a