Memory leak: MCP clients not closed before reassignment #8257

Description

When MCP clients are reassigned in MCP.add() or during OAuth finishAuth(), the existing client at that key is silently overwritten without being closed first.

Impact

• Open connections/sockets are orphaned
• Active event listeners accumulate
• Resources are never released
• Contributes to memory growth in long-running sessions

Location

packages/opencode/src/mcp/index.ts - two locations where s.clients[name] = result.mcpClient overwrites existing clients

Reproduction

1. Connect to an MCP server
2. Reconnect or complete OAuth flow
3. Previous client is leaked

Related

Part of #3013 (general memory issues)
Related to #7261 (MCP orphan processes)