JsonlCacheStore: a single malformed line permanently blocks store construction

[myselfsiddharth]

Follow-up from review of #93.

JsonlCacheStore.loadInto() (src/cache/store.ts) parses every line of pool.jsonl/tenant.jsonl at construction time with a bare JSON.parse(trimmed) — no try/catch:

private loadInto(file: string): void {
  if (!existsSync(file)) return;
  const text = readFileSync(file, "utf8");
  for (const line of text.split("\n")) {
    const trimmed = line.trim();
    if (!trimmed) continue;
    this.index(JSON.parse(trimmed) as CacheRow);
  }
}

The existing test (tolerates a trailing newline and blank lines) only covers benign whitespace. A genuinely truncated/partial last line — e.g. process killed mid-appendFileSync, or a disk-full condition — would throw out of the constructor. Because the file is append-only and the bad line is never removed, this isn't a transient failure: every future new JsonlCacheStore({dir}) against that directory throws, permanently blocking the cache until someone manually edits the file.

Not urgent to fix right now: nothing outside src/cache/ and tests constructs a JsonlCacheStore yet (architecture.md stub 7 — the cache read/write path isn't wired into the runtime), so this can't happen in production today. Worth a regression test + a decision (skip-and-warn the bad line vs. surface a clear "corrupt cache, delete <file> to reset" error) before the cache store is wired into a live path.