Closes #49

What

Adds TTLTracker to pkg/cache. Monitors Anthropic's 5-minute prompt cache TTL per prefix hash, detects cold-start penalties, and tells batch jobs the latest safe time to send the next request without letting the cache expire.

Changes

pkg/cache/ttl.go (new)

• AnthropicCacheTTL constant (5 minutes)
• TTLTracker: tracks TTLEntry per prefix hash behind sync.Mutex
• Touch(prefixHash): records request, returns wasAlive (warm hit vs cold start); refreshes ExpiresAt on every call
• NextDeadline / TimeUntilExpiry: time remaining before cache expires
• ScheduleDeadline(hash, safetyMargin): latest safe time for next batch request (ExpiresAt - margin)
• ExpiredEntries(): all prefixes past their TTL deadline
• Evict(hash): remove entry on boundary retreat
• Stats(): aggregate AlivePrefixes, ExpiredPrefixes, TotalHits, TotalMisses

pkg/cache/ttl_test.go (new) — 9 tests

Usage

tracker := cache.NewTTLTracker(0) // 0 = AnthropicCacheTTL (5 min)

// After each request with a cache_control marker:
wasAlive := tracker.Touch(plan.PrefixHash)
if !wasAlive {
    metrics.RecordCacheBoundary(sessionID, 0, false, true) // boundary retreated
}

// Batch scheduling: sleep until just before cache expires
deadline := tracker.ScheduleDeadline(plan.PrefixHash, 30*time.Second)
time.Sleep(time.Until(deadline))