Session JSON messages lack timestamp field for proper ordering

Problem

Session JSON files stored in ~/.hermes/sessions/session_*.json do not include a timestamp field on individual message objects. The messages database table has a NOT NULL constraint on the timestamp column.

When messages are recovered from JSON (e.g., after a crash), timestamps must be estimated proportionally from session_start and last_updated fields — making accurate message ordering impossible.

Impact

• Cannot restore accurate message ordering across concurrent sessions
• Cannot determine which messages were created within the same second
• Time-based queries on session history return estimated values, not actual

Proposed Fix

Add a timestamp field (Unix epoch, integer or float) to each message object in the JSON file, populated at message creation time.

This is a non-breaking change:

• Existing JSON files without timestamp fall back to proportional estimation
• New JSON files written by Hermes include the field automatically
• SessionDB JSON import logic already handles missing fields gracefully

Schema Change

Message objects in session_*.json should include:

{
  "role": "user",
  "content": "...",
  "timestamp": 1748301612.5,
  "tool_call_id": "..."
}

Files Likely Involved

• Session JSON serialization/deserialization code (where messages are written to sessions/ directory)
• SessionDB JSON import logic in hermes_agent/hermes_state.py