Phase 008: close the test and CI loop around the migration

Adds the checks that make the constraints from Phase 001-007 mechanical
instead of tribal knowledge, and clears the legacy brand text that was still
reachable at runtime.

New red-line tests:
  - no-legacy-brand mirrors the harness scan under vitest, and asserts its
    allow-list has not drifted from harness-check.mjs
  - hermes-db-readonly classifies every new DatabaseSync call site by
    following the path it opens, and fails if a state.db handle drops
    readOnly. It also pins the one legitimately writable database.
  - i18n-parity compares the raw zh/en locale objects both ways. The
    existing coverage test merged en into zh at import time, so a key
    missing from zh could never fail there.

harness:check gains the four missing gates from the phase brief - retired
package directories, retired package scripts, a single UI library, and the
required root files. Each was verified to fail by mutating the repo.

The online-update channel had hardcoded upstream URLs. It now reads
HERMES_WEB_UI_VERSION_MANIFEST_URL and friends and degrades with a named
error, because a reimplementation has nowhere to pull releases from.

The remaining legacy strings are deliberate and each carries a reason in the
allow-list: the global-agent relay address and the OpenRouter referer are
load-bearing, the socket role identifier is a wire contract, and the pre-rename
DB filenames would orphan existing deployments if changed. AGENTS.md now says
so explicitly, so a future agent does not "clean them up".

Also removes a dead electron mock alias, an i18n exemption that had stopped
exempting anything, and a teardown race in plugins-view.test.ts where a-spin
animation frames outlived the jsdom environment.

Verified self-contained: with reference_code/ moved aside, harness:check,
build, test and test:e2e all pass, so the directory can now be deleted.

tests/client/i18n-coverage.test.ts

@@ -197,10 +197,11 @@ function labelLength(value: unknown): number {
 }
 
 describe('i18n locale coverage', () => {
-  const ALLOWED_MISSING_KEYS = new Set([
-    'changelog.new_0_5_4_7',
-    'chat.sessionNotFound',
-  ])
+  // The changelog treats each release's change blurb as a key, but those live
+  // in packages/client/src/data/changelog.ts, not in the locale files.
+  // chat.sessionNotFound used to be exempt here too, but it is defined in both
+  // locales, so the exemption only hid real gaps.
+  const ALLOWED_MISSING_KEYS = new Set(['changelog.new_0_5_4_7'])
 
   beforeAll(() => {
     for (const l of supportedLocales) {