Feature Request: Support multiple simultaneous memory providers

Problem

Hermes currently only supports one external memory provider at a time (memory.provider in config.yaml). Different providers excel at different things:

They complement each other perfectly, but you can only pick one.

Real-world example

A user (physics teacher, macOS) has:

• Structured facts: PDF page offsets, TTS engine specs, tool paths — perfect for fact_store's reason(entities=['PDF', 'physics']) queries
• Session history: past conversations, project progress — perfect for viking_search(mode='fast') semantic recall

Switching to OpenViking kills fact_store access. Switching to Holographic kills session search. Neither is ideal.

Proposed solution

Allow multiple memory providers to coexist:

memory:
  memory_enabled: true
  providers:
    openviking:
      enabled: true
    holographic:
      enabled: true

Or at minimum, allow fact_store to always be available alongside any external provider, given it's pure local SQLite with zero runtime overhead.

Why this matters

The current single-provider limitation forces users to choose between complementary capabilities. For users running fully local setups (OpenViking + Holographic = zero additional infra), this is an unnecessary constraint — both can and should coexist.