fix(session-resume): address review findings for PR #98

- Derive conversation anchor from array content and return undefined when no usable user message.
- Add contentPrefix + tool/subagent fingerprint checks to resume cache lookups.
- Make failure-path cache eviction resume-specific via isResumeSpecificFailure.
- Add LRU refresh on read/write and TTL expiry logging.
- Add typed ProxyMessage/ContentPart unions and ResolvedPrompt interface.
- Expand tests for anchor, fingerprint mismatches, multimodal fallback, isResumeSpecificFailure, and plugin resume orchestration.