feat(ms22-p2): add Discord channel → agent routing

- Add DISCORD_AGENT_CHANNELS env var for channel-to-agent mapping
- Route messages in agent channels to specific agents via ChatProxyService
- Handle SSE streaming responses and Discord's 2000 char limit
- Add workspace owner lookup for chat proxy authentication
- Add comprehensive unit tests for agent channel routing

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.env.example

@@ -343,6 +343,11 @@ RATE_LIMIT_STORAGE=redis
 # DISCORD_CONTROL_CHANNEL_ID=channel-id-for-commands
 # DISCORD_WORKSPACE_ID=your-workspace-uuid
 #
+# Agent channel routing: Maps Discord channels to specific agents.
+# Format: <channelId>:<agentName>,<channelId>:<agentName>
+# Example: 123456789:jarvis,987654321:builder
+# DISCORD_AGENT_CHANNELS=
+#
 # SECURITY: DISCORD_WORKSPACE_ID must be a valid workspace UUID from your database.
 # All Discord commands will execute within this workspace context for proper
 # multi-tenant isolation. Each Discord bot instance should be configured for