F2 SLICE-1: artifact schema + CoauthorStore with round-trip tests (#4)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/store.ts

@@ -0,0 +1,31 @@
+/**
+ * CoauthorStore — load/save the per-document `.threads/` sidecar (spec §6.4).
+ * Git-native, serverless, plain pretty JSON (INV-2). vscode-free (Node fs only),
+ * so it is unit-testable; the FileSystemWatcher that triggers re-anchoring on
+ * external change is wired in the vscode layer (ThreadController, SLICE-4).
+ */
+import * as fs from "node:fs";
+import * as path from "node:path";
+import { serializeArtifact, type Artifact } from "./model";
+
+export class CoauthorStore {
+  /** @param rootDir absolute workspace-folder root that owns the `.threads/` tree. */
+  constructor(private readonly rootDir: string) {}
+
+  /** `.threads/<repo-relative-docPath>.json` (spec §6.3). */
+  sidecarPath(docPath: string): string {
+    return path.join(this.rootDir, ".threads", `${docPath}.json`);
+  }
+
+  load(docPath: string): Artifact | null {
+    const p = this.sidecarPath(docPath);
+    if (!fs.existsSync(p)) return null;
+    return JSON.parse(fs.readFileSync(p, "utf8")) as Artifact;
+  }
+
+  save(docPath: string, artifact: Artifact): void {
+    const p = this.sidecarPath(docPath);
+    fs.mkdirSync(path.dirname(p), { recursive: true });
+    fs.writeFileSync(p, serializeArtifact(artifact), "utf8");
+  }
+}