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-agent-sock/README.md

@@ -14,24 +14,25 @@ Unlike the host-served sockets, this one **never leaves the container**.
 - the loose-ends-v2 **todo** op family
 - the harness-local **reminder** op family
 
-More in-agent request families may be added over time — the socket is
+More in-agent request families may show up over time — the socket is
 deliberately named for the agent, not for the todos.
 
 **Why in-container**: the harness owns the todo + reminder stores locally and
-signals its own turn loop directly, so `hive-c0re` is not in either path: no
+signals its own turn loop directly, so `hive-c0re` isn't in either path: no
 broker round-trip, no long-poll, no marker files. That locality is the
 point — it's also what lets these stores travel with the agent for hive
 portability.
 
-**Not to be confused with `hive-core-agent-sock`**: that crate is the
+**Don't confuse this with `hive-core-agent-sock`**: that crate is the
 _host_-served core↔agent protocol on `/run/hive/mcp.sock` (the harness
 talking out to `hive-c0re`). This socket is purely intra-container.
 
-## `extra_mcp`: the `hyperhive.extraMcpServers` spec
+## `extra_mcp`: the `services.hyperhive.agent.extraMcpServers` spec
 
 Not a socket protocol — a shared config type. Parses
-`/etc/hyperhive/extra-mcp.json` (the nix-rendered `hyperhive.extraMcpServers`
-option) and renders each entry into the shape claude expects in a
+`/etc/hyperhive/extra-mcp.json` (the nix-rendered
+`services.hyperhive.agent.extraMcpServers` option) and renders each
+entry into the shape claude expects in a
 `--mcp-config` blob. Shared by `hive-agent` (the main session's servers) and
 `hive-subagent-mcp` (the subagent-eligible subset, gated on
 `availableToSubagents`) — both already depend on this crate for the socket