feat(jsonl-store): shared audited JSONL plumbing (injection-reject + atomic append + tolerant read)

Single source of truth extracted for D2A: gstack-learnings-* and the upcoming
gstack-decision-* bins share one injection-pattern list, one atomic single-line
appender, and one tolerant reader. No more drift between stores.

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

lib/jsonl-store.ts

@@ -0,0 +1,93 @@
+/**
+ * jsonl-store — shared, audited plumbing for gstack's append-only JSONL stores.
+ *
+ * Single source of truth for the three things every JSONL store must get right:
+ *   1. Injection sanitization (the prompt-injection patterns that must NOT survive
+ *      into agent context when a record is later resurfaced).
+ *   2. Atomic single-line append (concurrent agents must not corrupt the file).
+ *   3. Tolerant read (a partially-written tail or one corrupt line must not take
+ *      down the whole read).
+ *
+ * Extracted from `bin/gstack-learnings-log` (D2A) so `gstack-learnings-*` and the
+ * new `gstack-decision-*` bins share ONE audited path — a new injection pattern or
+ * a write-atomicity fix lands in both at once, never drifts. Per the
+ * `squash-with-regen` / DRY discipline + the eng-review D2A decision.
+ */
+
+import { appendFileSync, readFileSync, existsSync } from "fs";
+
+/**
+ * Prompt-injection patterns. If any matches a free-text field (insight, rationale,
+ * decision), the record is REJECTED at write time — these strings could otherwise
+ * be replayed into a future agent's context as instructions when the record is
+ * resurfaced. Keep this list the ONLY copy (callers import it; do not re-declare).
+ */
+export const INJECTION_PATTERNS: readonly RegExp[] = [
+  /ignore\s+(all\s+)?previous\s+(instructions|context|rules)/i,
+  /you\s+are\s+now\s+/i,
+  /always\s+output\s+no\s+findings/i,
+  /skip\s+(all\s+)?(security|review|checks)/i,
+  /override[:\s]/i,
+  /\bsystem\s*:/i,
+  /\bassistant\s*:/i,
+  /\buser\s*:/i,
+  /do\s+not\s+(report|flag|mention)/i,
+  /approve\s+(all|every|this)/i,
+];
+
+/** True if `text` contains an instruction-like injection pattern. */
+export function hasInjection(text: string): boolean {
+  return INJECTION_PATTERNS.some((p) => p.test(text));
+}
+
+/** Returns the first injection pattern that matches, or null. For actionable errors. */
+export function firstInjectionMatch(text: string): RegExp | null {
+  return INJECTION_PATTERNS.find((p) => p.test(text)) ?? null;
+}
+
+/**
+ * Atomic single-line append of `obj` as one JSON line.
+ *
+ * Concurrency: opens with `a` (O_APPEND); a single write under PIPE_BUF (>=512,
+ * 4096+ on macOS/Linux) is atomic across processes, so concurrent agents appending
+ * never interleave. Records MUST serialize to a single line (no embedded newline) —
+ * we throw rather than risk a multi-line record breaking the one-record-per-line
+ * invariant the tolerant reader relies on.
+ *
+ * Caveat: a record larger than PIPE_BUF loses the cross-process atomicity guarantee.
+ * Keep records line-bounded; very large free-text should be truncated by the caller.
+ */
+export function appendJsonl(path: string, obj: unknown): void {
+  const line = JSON.stringify(obj);
+  if (line.includes("\n")) {
+    throw new Error("jsonl-store: record serialized to multiple lines (embedded newline)");
+  }
+  appendFileSync(path, line + "\n", { encoding: "utf-8" });
+}
+
+/**
+ * Tolerant reader: parse each line, SKIP malformed ones (partial-write tail, a
+ * corrupt line, a non-JSON line) rather than throwing. A broken line never takes
+ * down the whole read. Missing file → empty array. Unknown fields are preserved
+ * (forward-compatible: a schema bump on the writer doesn't break older readers).
+ */
+export function readJsonl<T = unknown>(path: string): T[] {
+  if (!existsSync(path)) return [];
+  let raw: string;
+  try {
+    raw = readFileSync(path, "utf-8");
+  } catch {
+    return [];
+  }
+  const out: T[] = [];
+  for (const line of raw.split("\n")) {
+    const trimmed = line.trim();
+    if (!trimmed) continue;
+    try {
+      out.push(JSON.parse(trimmed) as T);
+    } catch {
+      // Malformed line (partial tail / corruption) — skip, keep reading.
+    }
+  }
+  return out;
+}