Each agent now maintains its own session cache file instead of sharing
a single agent-sessions.json. This prevents session ID conflicts when
multiple agents operate on the same thread+role pair.
Changes:
- getCachePath() now takes agentName parameter
- getCachedSessionId/setCachedSessionId require agentName as first param
- Cache files named <agent>-sessions.json (e.g., hermes-sessions.json)
- Agent wrappers inject their agent name into cache calls
- Add comprehensive tests for session cache isolation
- Handle malformed JSON gracefully (treat as empty cache)
Fixes#461
