chore(config): remove example MCP servers from default config

Remove pre-populated example servers (filesystem, github, brave-search,
postgres) from DefaultConfig() to reduce memory footprint per instance.

Changes:
- Set MCP.Servers to empty map instead of 4 example servers
- Reduces default config size by ~500 bytes per instance
- Users should add MCP servers via config.json or documentation

Example configurations are still available in:
- README.md MCP section
- config.example.json
- Official MCP documentation

This optimization benefits deployments with many agent instances.

pkg/config/config.go

@@ -353,35 +353,7 @@ func DefaultConfig() *Config {
 			},
 			MCP: MCPConfig{
 				Enabled: false,
-				Servers: map[string]MCPServerConfig{
+				Servers: map[string]MCPServerConfig{},
-					"filesystem": {
-						Enabled: false,
-						Command: "npx",
-						Args:    []string{"-y", "@modelcontextprotocol/server-filesystem", "/tmp"},
-						Env:     map[string]string{},
-					},
-					"github": {
-						Enabled: false,
-						Command: "npx",
-						Args:    []string{"-y", "@modelcontextprotocol/server-github"},
-						Env: map[string]string{
-							"GITHUB_PERSONAL_ACCESS_TOKEN": "YOUR_GITHUB_TOKEN",
-						},
-					},
-					"brave-search": {
-						Enabled: false,
-						Command: "npx",
-						Args:    []string{"-y", "@modelcontextprotocol/server-brave-search"},
-						Env: map[string]string{
-							"BRAVE_API_KEY": "YOUR_BRAVE_API_KEY",
-						},
-					},
-					"postgres": {
-						Enabled: false,
-						Command: "npx",
-						Args:    []string{"-y", "@modelcontextprotocol/server-postgres", "postgresql://user:password@localhost/dbname"},
-					},
-				},
 			},
 		},
 		Heartbeat: HeartbeatConfig{