Prompt-cache diagnostics track a 'prefix-break hash' across sessions and forks

What

• The per-request cache-state tracker now computes and stores a prefixBreakHash (when hash-sample messages are available), used alongside existing message-hash tracking to help detect changes that break prompt caching. It also tracks a summaryForks map recording parent/child cache state around session summarization and forking (capped at a limit), and reports cache-break telemetry (cacheBroke, changeKinds).
• When restoring cached session state from disk, entries now compute a hydratedHashRows value from a stored prefixBreakHash when its salt is recognized, and reset callCount/cacheDeletionsPending on restore.
• The cache-diagnostics tracker also accepts a baselineAgentId, and when a forked agent has no prior baseline of its own, it seeds its baseline from its parent's cache state (forkAgentId/parentKey) instead of starting cold, recording where the baseline came from (baselineHashSource: memory, disk, or none).

Why

This gives better diagnostics for why prompt caching breaks, and it means forked or restored sessions no longer lose their cache-diagnostic history, reducing false alarms about the cache having broken.