feat: add session rollover lifecycle helper

[Tosko4]

Summary

• add a small rollover_session(...) lifecycle helper to the LCM engine
• cover repeated /new-style session boundaries without duplicating retained nodes
• tighten Hermes-native session rollover behavior without pretending the engine already has full transcript bootstrap / rotation machinery

Why

Session boundaries are exactly where long-lived context engines get weird if lifecycle handling is loose.

This PR takes the smallest useful step:

• flush old-session state
• rebind to the new session
• optionally carry retained summaries forward
• make repeated rollover boundaries deterministic

That gives Hermes LCM a cleaner rollover path right now, without overreaching into a much larger bootstrap/checkpoint design in the same slice.

Notes

This is intentionally not a full bootstrap / checkpoint / transcript-rotation implementation yet.
That larger state model still needs its own design and review.

Related host-side compatibility follow-up: NousResearch/hermes-agent#8416.
That companion PR improves Hermes Agent's external context-engine lifecycle ergonomics without changing default built-in behavior or bundling hermes-lcm into Hermes Agent.

Test Plan

• python3 -m pytest tests/test_lcm_engine.py -q
• python3 -m pytest tests/ -q
• local host dogfood check confirmed Hermes is loading this plugin branch from ~/.hermes/plugins/hermes-lcm

[stephenschoettler]

Simple and correct. Having the lifecycle ordering (flush → reset → bind → carry over) in one method prevents integration bugs. Repeated rollover test is a good edge case to cover.