fix: address review round 4 — simplify parallel(), add Sentry cache spans

Review comments addressed:

#1: issue/list.ts FRESH_ALIASES — verified no conflict, only log/list.ts
    has f: 'follow' and it doesn't use FRESH_ALIASES
#2: cacheHeuristic 0.1 vs CLEANUP_PROBABILITY — different semantics,
    not shared (RFC heuristic vs probabilistic cleanup trigger)
#3: Simplified parallel() — replaced custom wrapper with p-limit's
    built-in .map() method (cacheIO.map(items, fn)) at all 3 call sites
#4: evictExcessEntries/deleteExpiredEntries — run both in parallel via
    Promise.all() since they operate on disjoint file sets
#5: Added Sentry instrumentation — withCacheSpan() helper in telemetry.ts,
    cache.lookup and cache.store spans in response-cache.ts with URL attrs

Author: BYK