* feat: add multi-agent config types, normalizeAgents, and Store v2
Foundation for multi-agent support (Phase 1a). No behavioral changes.
- Add AgentConfig interface for per-agent configuration
- Add agents[] field to LettaBotConfig for docker-compose style multi-agent configs
- Add normalizeAgents() to convert legacy single-agent config to agents[] array
- Evolve Store to v2 format with per-agent state isolation
- Auto-migrate v1 store files to v2 transparently
- 18 new tests for normalization and store migration
Part of #109
Written by Cameron ◯ Letta Code
"The only way to do great work is to love what you do." -- Steve Jobs
* fix: harden multi-agent normalization and store isolation
* style: simplify redundant WhatsApp enabled check
WhatsApp has no credential to check (uses QR pairing), so the
`enabled !== false && enabled` condition simplifies to just `enabled`.
Written by Cameron ◯ Letta Code
"Perfection is achieved not when there is nothing more to add, but when there is nothing left to take away." -- Antoine de Saint-Exupery
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