Bug: on_pre_compress() return value discarded + no summary persistence

Problem

When Hermes compacts context, two issues cause context loss:

1. on_pre_compress() return value ignored — MemoryManager.on_pre_compress() collects insights from external memory providers (ByteRover, etc.) before context is compressed, but _compress_context() in run_agent.py calls it without capturing the return value. Provider insights are gathered but never injected into the compaction summary.

2. No summary disk persistence — The compaction summary is held in memory (_previous_summary) for iterative updates within a session, but never written to disk. If the session restarts or crashes, the summary is lost entirely. There's no mechanism to recover context across sessions.

Changes

run_agent.py:

• Capture on_pre_compress() return value into provider_context variable
• Thread provider_context through to compress() → _generate_summary()
• After compression, persist the summary to ~/.hermes/sessions/compaction_summary_{session_id}.md

agent/context_compressor.py:

• compress() accepts new provider_context parameter (backward-compatible, defaults to "")
• _generate_summary() accepts new provider_context parameter
• When provider context is non-empty, it's injected into the LLM summary prompt as "EXTERNAL MEMORY PROVIDER CONTEXT" section

Testing

40/40 context_compressor tests pass
46/46 memory provider tests pass
1117/1122 agent tests pass (5 pre-existing failures unrelated to this change)

Impact

• External memory providers' pre-compression insights now actually survive compaction
• Summaries are persisted to disk for cross-session recovery
• Fully backward-compatible — no behavior change when no providers are registered