refactor: rewrite on @punktfunk/plugin-kit — contract + plugin workspaces

The framework half of this plugin (engine lifecycle, config/state, host
reconcile, UI server, CLI) was ~80% identical to rom-manager's. That code is
now @punktfunk/plugin-kit; this replaces the local copy with it and adopts the
rom-manager blueprint layout.

  contract/  Effect Schemas: the cross-process export contract (the C#
             exporter's other half, with the schema-version guard as a decode
             check), the config schema (one schema, Encoded = authored file
             shape, defaults only via withDecodingDefaultKey), domain DTOs, the
             HttpApi contract, API errors.
  plugin/    src/domain = the pure core (locate/read, art, reconcile), ported
             tests green before any Effect wiring; src/services = the kit's
             ConfigService/CacheStore/SyncEngine/ProviderClient; definePluginKit
             entry, kit CLI, auth-free dev API.

Preserved, because they are what makes playnite work at all: the ingest inbox
is still read FIRST (the de-privileged LocalService runner cannot reach the
interactive user's %APPDATA%), the host/dataurl/off art modes, and the
playnite:// launch hand-back. The C# exporter is untouched.

New: per-game include/exclude overrides, and an allow-listed /api/art proxy so
the SPA can render local Playnite covers — it serves only paths the currently
ingested export references.

Dropped: the standalone password UI server (console surface + CLI only, as in
rom-manager 0.3.0) and the devEntry flag.

Trap found and fixed here: the HttpApi encoder serialises `undefined` as
`null`, so a `Schema.optional` field the server omits cannot be decoded by the
client — silently in the SSE feed, loudly in the typed client. Every optional
EngineStatus field is `Schema.NullOr` with an explicit value on the wire.

contract/src/errors.ts

@@ -0,0 +1,25 @@
+// API errors — Schema-backed so they cross the wire typed; status set per-endpoint via
+// HttpApiSchema.status in api.ts.
+import { Schema } from "effect";
+
+/** PUT /api/config body failed schema validation. → 400 */
+export class ConfigInvalid extends Schema.TaggedErrorClass<ConfigInvalid>()(
+	"ConfigInvalid",
+	{ issues: Schema.String },
+) {}
+
+/** A sync pass is already running (the trigger was coalesced). → 409 */
+export class SyncInProgress extends Schema.TaggedErrorClass<SyncInProgress>()(
+	"SyncInProgress",
+	{},
+) {}
+
+/**
+ * No exporter output was found, or the file is unreadable / not a valid export / stamped
+ * with a schema this plugin does not understand. → 503, because it is a "the other half
+ * isn't installed yet" state rather than a server fault: the UI turns it into onboarding.
+ */
+export class ExportUnavailable extends Schema.TaggedErrorClass<ExportUnavailable>()(
+	"ExportUnavailable",
+	{ message: Schema.String },
+) {}