Phase 15: add B-003 regression script with unit gate and artifact audit

Run compaction fallback, kept inject, and mid-turn steer tests; document
lost-reinject.jsonl auto→null source signature from the original bug.

Co-authored-by: Cursor <cursoragent@cursor.com>

scripts/b003-repro.mjs

@@ -0,0 +1,82 @@
+#!/usr/bin/env node
+/**
+ * Phase 15 regression gate for B-003 (mid-turn / missed reinject).
+ *
+ * 1. Runs focused unit tests for source fallback, kept inject entries, mid-turn steer.
+ * 2. Audits lost-reinject.jsonl (if present) for the pre-fix failure signature.
+ *
+ * Full two-compact RPC E2E needs a long session; use manual checklist in TODO.md §15.
+ */
+import { spawnSync } from "node:child_process";
+import { existsSync, readFileSync } from "node:fs";
+import { fileURLToPath } from "node:url";
+import { resolve, dirname } from "node:path";
+
+const repoRoot = resolve(dirname(fileURLToPath(import.meta.url)), "..");
+const artifactPath = resolve(repoRoot, "lost-reinject.jsonl");
+
+const unitPatterns = [
+	"test/compaction-source-fallback.test.ts",
+	"test/kept-window.test.ts",
+	"test/reinject-mid-turn.test.ts",
+	"test/reinject-double-compact.test.ts",
+];
+
+function runUnitGate() {
+	const result = spawnSync("npm", ["test", "--", ...unitPatterns], {
+		cwd: repoRoot,
+		stdio: "inherit",
+	});
+	if (result.status !== 0) {
+		process.exit(result.status ?? 1);
+	}
+}
+
+function auditLostReinjectArtifact() {
+	if (!existsSync(artifactPath)) {
+		console.log("b003-repro: lost-reinject.jsonl not found — skip artifact audit");
+		return;
+	}
+	const lines = readFileSync(artifactPath, "utf8").split("\n").filter(Boolean);
+	const stateEntries = [];
+	const injectEntries = [];
+	for (const line of lines) {
+		try {
+			const entry = JSON.parse(line);
+			if (entry.customType === "skill-reinject:state" && entry.data) {
+				stateEntries.push({
+					id: entry.id,
+					ts: entry.timestamp,
+					lastCompactionSource: entry.data.lastCompactionSource,
+					pendingReinject: entry.data.pendingReinject ?? [],
+				});
+			}
+			if (entry.type === "custom_message" && entry.customType === "skill-reinject:inject") {
+				injectEntries.push({ id: entry.id, ts: entry.timestamp });
+			}
+		} catch {
+			// skip malformed lines
+		}
+	}
+	const nullSourceAfterAuto = stateEntries.filter(
+		(entry, index) =>
+			index > 0 &&
+			entry.lastCompactionSource === null &&
+			stateEntries[index - 1]?.lastCompactionSource === "auto",
+	);
+	console.log("b003-repro artifact audit:");
+	console.log(`  state snapshots: ${stateEntries.length}`);
+	console.log(`  skill-reinject:inject messages: ${injectEntries.length}`);
+	console.log(`  auto→null source transitions (pre-fix bug): ${nullSourceAfterAuto.length}`);
+	if (nullSourceAfterAuto.length > 0) {
+		console.log("  sample null-source entry:", JSON.stringify(nullSourceAfterAuto[0]));
+		console.log(
+			"  post-fix expectation: ensureCompactionSourceMarked on session_compact → lastCompactionSource: auto",
+		);
+	}
+}
+
+console.log("b003-repro: running unit regression gate…");
+runUnitGate();
+auditLostReinjectArtifact();
+console.log("b003-repro: PASS (unit gate)");