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.

frontend/README.md

@@ -2,11 +2,13 @@
 
 npm workspaces project for the hyperhive browser-facing assets:
 
-- `packages/shared/` — shared modules used by both surfaces (terminal
+- `packages/shared/` — shared modules used by every surface (terminal
   pane, Catppuccin palette + body typography).
 - `packages/dashboard/` — the hive-c0re dashboard SPA.
 - `packages/agent/` — the per-container web UI (default agent page,
   stats, screen).
+- `packages/swarm-ui/` — the swarm-level operator UI shell (project-
+  bootstrap scope, no functionality yet).
 
 ## Build
 
@@ -16,18 +18,19 @@ npm run build          # builds every workspace into packages/*/dist/
 ```
 
 The Rust binaries serve `packages/dashboard/dist/` and
-`packages/agent/dist/` via `tower_http::ServeDir` at runtime; the
-build derivation is wired up in `nix/packages/frontend.nix`. Per-agent
-additions are layered on top of the default agent dist via the
-`hyperhive.frontend.extraFiles` option in `agent.nix`.
+`packages/agent/dist/` via `tower_http::ServeDir` at runtime;
+`nix/packages/frontend.nix` wires up the build derivation. The
+`services.hyperhive.agent.frontend.extraFiles` option in `agent.nix`
+layers per-agent additions on top of the default agent dist.
 
 ## Why npm + esbuild
 
 - **Hermetic**: dependencies vendored via the checked-in lockfile;
   `buildNpmPackage` in nix uses it as the source-of-truth so the
   output is reproducible without network access at build time.
-- **esbuild**: vanilla-JS bundler, no framework runtime overhead.
-  Each workspace's `build.mjs` is ~30 lines.
+- **esbuild**: each package is a small Preact app, bundled with no
+  framework beyond Preact itself. Each workspace's `build.mjs` is ~30
+  lines.
 - **Single-PR migration**: see the tracked design proposal and
   the four-commit shape (npm scaffold → nix derivations → container
   plumbing → Rust cutover).