refactor: consolidate duplicated plugin state and doctor migration plumbing onto SDK seams (#99850)

* refactor(plugin-sdk): add createPersistentDedupeCache and migrate channel presence caches

* refactor(matrix): adopt SDK approval reaction target store

* refactor(plugin-sdk): share doctor legacy-state migration fs helpers

* refactor(memory-core): dedupe qmd cache entry envelope validation

* chore(plugin-sdk): pin surface budgets for shared dedupe and doctor helpers

* test(matrix): use future approval expiry fixtures for reaction targets

* test(matrix): use future approval expiry fixtures for reaction targets

src/plugins/doctor-state-migration-fs.ts

@@ -0,0 +1,38 @@
+// Shared filesystem helpers for plugin doctor legacy-state migrations.
+import fs from "node:fs/promises";
+
+/** True when the legacy-state path exists and is a regular file. */
+export async function legacyStateFileExists(filePath: string): Promise<boolean> {
+  try {
+    const stat = await fs.stat(filePath);
+    return stat.isFile();
+  } catch {
+    return false;
+  }
+}
+
+/**
+ * Renames a migrated legacy source to `<path>.migrated`, recording the outcome in the
+ * doctor changes/warnings lists. Never throws: a failed archive leaves the source in
+ * place so a later doctor run can retry without losing migrated data.
+ */
+export async function archiveLegacyStateSource(params: {
+  filePath: string;
+  label: string;
+  changes: string[];
+  warnings: string[];
+}): Promise<void> {
+  const archivedPath = `${params.filePath}.migrated`;
+  if (await legacyStateFileExists(archivedPath)) {
+    params.warnings.push(
+      `Left migrated ${params.label} source in place because ${archivedPath} already exists`,
+    );
+    return;
+  }
+  try {
+    await fs.rename(params.filePath, archivedPath);
+    params.changes.push(`Archived ${params.label} legacy source -> ${archivedPath}`);
+  } catch (err) {
+    params.warnings.push(`Failed archiving ${params.label} legacy source: ${String(err)}`);
+  }
+}