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.

docs/process/gotchas.md

@@ -333,7 +333,7 @@ CI on drift).
 system-prompt template + claude-settings JSON as its own derivation,
 separate from the hive-ag3nt / hive-c0re crates. Reason: when the
 rust build's `src` was the whole repo tree, any tweak to
-`branding/agent-configs.svg` or `hive-ag3nt/prompts/system.md`
+`branding/agent-configs.svg` or `hive-agent/prompts/system.md`
 invalidated the cargo cache and forced a full rebuild. crane (and
 naersk before it) couldn't see "these inputs are unused by rust" on
 its own — the split breaks the coupling at the derivation boundary.