docs(readmes): fix stale facts in remaining crate READMEs + gotchas

- frontend/README.md: list the missing packages/swarm-ui package, fix
  the vanilla-JS claim (all packages depend on preact), and the
  deprecated hyperhive.frontend.extraFiles spelling
- hive-c0re/README.md: hive-c0re no longer provisions per-agent
  forge/matrix accounts (swarm-controller does); it wires gateway
  vhosts and reconciles forge/matrix config
- hive-metric/README.md: OTEL_EXPORTER_OTLP_HEADERS is never set by
  the harness and has no way to be set
- hive-agent-sock/README.md: fix the deprecated
  hyperhive.extraMcpServers spelling
- docs/process/gotchas.md: fix a dangling hive-ag3nt/ path, the real
  directory is hive-agent/

Also fixes pre-existing vale error-level alerts (passive voice,
Microsoft.Auto, Microsoft.Contractions) in the same files so
prose-lint-errors passes clean.

hive-metric/README.md

@@ -15,12 +15,15 @@ scripts use it to record per-agent metrics.
 hive-metric <name> <value> [--type counter|gauge] [--labels key=value ...]
 ```
 
-Standard OTEL environment variables are read automatically by the SDK:
+The SDK reads standard OTEL environment variables automatically:
 
 - `OTEL_EXPORTER_OTLP_ENDPOINT` — collector URL (**required**)
-- `OTEL_EXPORTER_OTLP_HEADERS` — auth headers (`Key=Value,...`)
 - `OTEL_RESOURCE_ATTRIBUTES` — resource labels (`k=v,...`)
 
-The harness populates all of these per-agent when
-`services.hyperhive.otel.enable = true`. See `docs/scheduler/observability.md` for the
-collector setup and the metrics hyperhive exports.
+The harness populates both per-agent when
+`services.hyperhive.otel.enable = true`. There's no
+`OTEL_EXPORTER_OTLP_HEADERS` and no way to add one: an agent exports
+straight to its hive's own collector and holds no upstream credential
+to put there (`nix/agent-modules/otel.nix`). See
+`docs/scheduler/observability.md` for the collector setup and the
+metrics hyperhive exports.