Add public API for MCP server installation (#122)

 Add a new public package `pkg/mcpinstall` that exposes MCP server
  installation functionality for use by external projects.

  Public API:
  - Install(opts Options) - Install MCP server config for a client
  - SupportedClients() - List available clients with names and IDs
  - Options struct - All configuration in one place
  - ClientInfo struct - Client display name and identifier

  Options fields:
  - ClientName (required) - Client identifier (e.g., "claude-code", "cursor")
  - ServerName (required) - Name to register the MCP server as
  - Command (required) - Path to the MCP server binary
  - Args (required) - Arguments to pass to the binary
  - CreateBackup (optional) - Create backup before modification
  - CustomConfigPath (optional) - Override default config location

  Example usage:
      err := mcpinstall.Install(mcpinstall.Options{
          ClientName: "cursor",
          ServerName: "my-mcp-server",
          Command:    "/usr/local/bin/my-server",
          Args:       []string{"serve"},
      })

  Internal changes:
  - Refactor InstallMCPForClient to accept configurable options
  - Separate Tiger-specific defaults into installTigerMCPForClient
  - Fix VSCode buildInstallCommand to marshal entire JSON struct
  - Simplify tests to pass values directly

Author: cevian

internal/tiger/cmd/mcp.go

@@ -107,7 +107,7 @@ Examples:
 				clientName = args[0]
 			}
 
-			return installMCPForClient(clientName, !noBackup, configPath)
+			return installTigerMCPForClient(clientName, !noBackup, configPath)
 		},
 	}