#8792: feat(feishu): add DM session isolation option

## Summary Add `dmSessionIsolation` config option to isolate DM sessions per sender. - When enabled, each DM sender gets their own session (separate conversation history) - Session key format: `agent:main:feishu:{accountId}:dm:{senderId}` - Includes accountId to avoid collisions across multiple Feishu accounts - Default: `false` (all DMs share the main session, preserving existing behavior) ## Configuration ```yaml channels: feishu: dmSessionIsolation: true # Each DM user gets isolated session ``` ## Test plan - [x] Unit tests added for session key resolution - [x] Tested on remote server 🤖 Generated with [Claude Code](https://claude.com/claude-code)