fix(mcp): user config overrides Claude Code .mcp.json with collision warning

Previously, Claude Code's .mcp.json would silently override OpenCode user
config when MCP server names collided. This was unexpected behavior since
users expect their explicit OpenCode configuration to take precedence.

Changes:
1. Swapped merge order: Claude Code .mcp.json is now merged BEFORE user
   config, so user config wins on collision
2. Added warning log when user config overrides a Claude Code MCP server:
   'warning: MCP server X from user config overrides Claude Code .mcp.json'
3. Added comprehensive tests for collision scenarios

Fixes #2946

src/plugin-handlers/mcp-config-handler.test.ts

@@ -164,4 +164,5 @@ describe("applyMcpConfig", () => {
     const mergedMcp = config.mcp as Record<string, Record<string, unknown>>
     expect(mergedMcp).not.toHaveProperty("plugin:custom")
   })
+
 })