hermes - 💡(How to fix) Fix Warn when memory store approaches memory_char_limit [1 pull requests]

Follow-up from #35121 and the production report in https://github.com/NousResearch/hermes-agent/issues/35121#issuecomment-4582762430.

Problem: Users currently discover memory pressure only after a write fails with quota exceeded. In production, a store at 2,196/2,200 chars caused repeated failed memory writes until operators manually raised memory.memory_char_limit, pruned stale entries, and added a local failure fuse. #35243 addresses the deterministic retry loop, but not proactive visibility.

Expected behavior:

• Surface memory usage when the store is near the configured memory_char_limit, for example at >=95%.
• Provide actionable guidance before the next write fails: prune stale entries, shorten proposed memory, replace an existing entry, or raise the configured limit if appropriate.
• Keep this as a warning, not a hard blocker.
• Document how operators should tune memory.memory_char_limit for long-lived agents.

Acceptance:

• A memory store at or above the warning threshold emits a structured warning with current size and configured limit.
• The warning appears in a user/operator-visible path before or alongside memory writes.
• Tests cover below-threshold, at-threshold, and over-limit cases.
• Existing quota-exceeded behavior and #35243 non-retryable guardrail behavior remain intact.